Rhodocene

<h2 id="history">History</h2>

Discoveries in <a href="/facts/Organometallic_chemistry/bhqFz2P0">organometallic chemistry</a> have led to important insights into <a href="/facts/Chemical_bonding/ArNUMnIB">chemical bonding</a>. <a href="/facts/Zeise%2527s_salt/DWyXJEh0">Zeise's salt</a>, K[PtCl3(C2H4)]·H2O, was reported in 1831<a class="footnote-ref" id="fnref:28" href="#fn:28">28</a> and <a href="/facts/Ludwig_Mond/RsJ2euMn">Mond's</a> discovery of <a href="/facts/Nickel_tetracarbonyl/8Py5Dre1">nickel tetracarbonyl</a> (Ni(CO)4) occurred in 1888.<a class="footnote-ref" id="fnref:29" href="#fn:29">29</a> Each contained a bond between a metal centre and small molecule, <a href="/facts/Ethylene/QMNBYLYn">ethylene</a> in the case of Zeise's salt and <a href="/facts/Carbon_monoxide/CKuLtEMJ">carbon monoxide</a> in the case of nickel tetracarbonyl.<a class="footnote-ref" id="fnref:30" href="#fn:30">30</a> The <a href="/facts/Space-filling_model/PcHWmvfV">space-filling model</a> of the anion of Zeise's salt (image at left)<a class="footnote-ref" id="fnref:31" href="#fn:31">31</a><a class="footnote-ref" id="fnref:32" href="#fn:32">32</a> shows direct bonding between the <a href="/facts/Platinum/qJC6lQf7">platinum</a> metal centre (shown in blue) and the carbon atoms (shown in black) of the ethylene <a href="/facts/Ligand/6etB3qeW">ligand</a>; such metal–carbon bonds are the defining characteristic of <a href="/facts/Organometallic_compound/bhqFz2P0">organometallic species</a>. Bonding models were unable to explain the nature of such metal–alkene bonds until the <a href="/facts/Dewar%25E2%2580%2593Chatt%25E2%2580%2593Duncanson_model/h7VTIqyV">Dewar–Chatt–Duncanson model</a> was proposed in the 1950s.<a class="footnote-ref" id="fnref:33" href="#fn:33">33</a><a class="footnote-ref" id="fnref:34" href="#fn:34">34</a><a class="footnote-ref" id="fnref:35" href="#fn:35">35</a> The original formulation covered only metal–alkene bonds<a class="footnote-ref" id="fnref:36" href="#fn:36">36</a> but the model was expanded over time to cover systems like <a href="/facts/Metal_carbonyl/KzJBcMzU">metal carbonyls</a> (including [Ni(CO)4]) where <a href="/facts/%25CE%25A0_backbonding/un6buomN">π backbonding</a> is important.<a class="footnote-ref" id="fnref:37" href="#fn:37">37</a>

<a href="/facts/Ferrocene/jKtu9Cp8">Ferrocene</a>, [Fe(C5H5)2], was first synthesised in 1951 during an attempt to prepare the <a href="/facts/Fulvalene/5zazI2Ap">fulvalene</a> (C10H8) by oxidative dimerization of <a href="/facts/Cyclopentadiene/S66eVYvh">cyclopentadiene</a>; the resultant product was found to have <a href="/facts/Molecular_formula/hghUtthl">molecular formula</a> C10H10Fe and reported to exhibit "remarkable stability".<a class="footnote-ref" id="fnref:38" href="#fn:38">38</a> The discovery sparked substantial interest in the field of organometallic chemistry,<a class="footnote-ref" id="fnref:39" href="#fn:39">39</a><a class="footnote-ref" id="fnref:40" href="#fn:40">40</a> in part because the structure proposed by <a href="/facts/Peter_Pauson/3z8Iz2m1">Pauson</a> and Kealy was inconsistent with then-existing bonding models and did not explain its unexpected stability. Consequently, the initial challenge was to definitively determine the structure of ferrocene in the hope that its bonding and properties would then be understood. The sandwich structure was deduced and reported independently by three groups in 1952: <a href="/facts/Robert_Burns_Woodward/bAQeInCb">Robert Burns Woodward</a> and <a href="/facts/Geoffrey_Wilkinson/BQ3GXMga">Geoffrey Wilkinson</a> investigated the reactivity in order to determine the structure<a class="footnote-ref" id="fnref:41" href="#fn:41">41</a> and demonstrated that ferrocene undergoes similar reactions to a typical aromatic molecule (such as <a href="/facts/Benzene/Fguue7nr">benzene</a>),<a class="footnote-ref" id="fnref:42" href="#fn:42">42</a> <a href="/facts/Ernst_Otto_Fischer/v8zCR6Ke">Ernst Otto Fischer</a> deduced the sandwich structure and also began synthesising other <a href="/facts/Metallocene/AecA5Qrk">metallocenes</a> including <a href="/facts/Cobaltocene/2aELFlSc">cobaltocene</a>;<a class="footnote-ref" id="fnref:43" href="#fn:43">43</a> Eiland and Pepinsky provided <a href="/facts/X-ray_crystallography/WtI0F5DN">X-ray crystallographic</a> confirmation of the sandwich structure.<a class="footnote-ref" id="fnref:44" href="#fn:44">44</a> Applying <a href="/facts/Valence_bond_theory/F04YPkfJ">valence bond theory</a> to ferrocene by considering an Fe2+ centre and two cyclopentadienide anions (C5H5−), which are known to be <a href="/facts/Aromaticity/wK0DzRY6">aromatic</a> according to <a href="/facts/H%25C3%25BCckel%2527s_rule/5BDPHGnk">Hückel's rule</a> and hence highly stable, allowed correct prediction of the geometry of the molecule. Once <a href="/facts/Molecular_orbital_theory/VJ6Kg8bI">molecular orbital theory</a> was successfully applied, the reasons for ferrocene's remarkable stability became clear.<a class="footnote-ref" id="fnref:45" href="#fn:45">45</a>
The properties of cobaltocene reported by Wilkinson and Fischer demonstrated that the unipositive cobalticinium cation [Co(C5H5)2]+ exhibited stability similar to that of ferrocene itself. This observation is not unexpected given that the cobalticinium cation and ferrocene are <a href="/facts/Isoelectronic/SbzuaI05">isoelectronic</a>, although the bonding was not understood at the time. Nevertheless, the observation led Wilkinson and <a href="/facts/F._Albert_Cotton/bXuU1zma">F. Albert Cotton</a> to attempt the synthesis of rhodocenium<a class="footnote-ref" id="fnref:46" href="#fn:46">46</a> and iridocenium <a href="/facts/Salt_(chemistry)/lsSKwGpn">salts</a>.<a class="footnote-ref" id="fnref:47" href="#fn:47">47</a> They reported the synthesis of numerous rhodocenium salts, including those containing the <a href="/facts/Bromide/bHSfxGk1">tribromide</a> ([Rh(C5H5)2]Br3), <a href="/facts/Perchlorate/WCC53zS2">perchlorate</a> ([Rh(C5H5)2]ClO4), and <a href="/facts/Reinecke%2527s_salt/sWuFLzW7">reineckate</a> ([Rh(C5H5)2] [Cr(NCS)4(NH3)2]·H2O) anions, and found that the addition of dipicrylamine produced a compound of composition [Rh(C5H5)2] [N(C6H2N3O6)2].<a class="footnote-ref" id="fnref:48" href="#fn:48">48</a> In each case, the rhodocenium cation was found to possess high stability. Wilkinson and Fischer went on to share the 1973 <a href="/facts/Nobel_Prize_in_Chemistry/8Nb8Ajjy">Nobel Prize</a> for Chemistry "for their pioneering work, performed independently, on the chemistry of the organometallic, so called <a href="/facts/Sandwich_compound/uhupMGDU">sandwich compounds</a>".<a class="footnote-ref" id="fnref:49" href="#fn:49">49</a><a class="footnote-ref" id="fnref:50" href="#fn:50">50</a>
The stability of metallocenes can be directly compared by looking at the <a href="/facts/Reduction_potential/PqpNUgSl">reduction potentials</a> of the one-electron <a href="/facts/Reduction_(chemistry)/Pyd5RVcj">reduction</a> of the unipositive cation. The following data are presented relative to the <a href="/facts/Saturated_calomel_electrode/5Lxe83Sk">saturated calomel electrode</a> (SCE) in <a href="/facts/Acetonitrile/fNpbvRp6">acetonitrile</a>:

[Fe(C5H5)2]+ / [Fe(C5H5)2] +0.38 V<a class="footnote-ref" id="fnref:51" href="#fn:51">51</a>
[Co(C5H5)2]+ / [Co(C5H5)2] −0.94 V<a class="footnote-ref" id="fnref:52" href="#fn:52">52</a>
[Rh(C5H5)2]+ / [Rh(C5H5)2] −1.41 V<a class="footnote-ref" id="fnref:53" href="#fn:53">53</a>
These data clearly indicate the stability of neutral ferrocene and the cobaltocenium and rhodocenium cations. Rhodocene is ca. 500 mV more reducing than cobaltocene, indicating that it is more readily oxidised and hence less stable.<a class="footnote-ref" id="fnref:54" href="#fn:54">54</a> An earlier <a href="/facts/Polarography/3JuFonc6">polarographic</a> investigation of rhodocenium perchlorate at neutral <a href="/facts/PH/RnSv8D7y">pH</a> showed a cathodic wave peak at −1.53 V (versus SCE) at the <a href="/facts/Dropping_mercury_electrode/wFs0j3sC">dropping mercury electrode</a>, corresponding to the formation rhodocene in solution, but the researchers were unable to isolate the neutral product from solution. In the same study, attempts to detect <a href="/facts/Iridocene/2f10n7ya">iridocene</a> by exposing iridocenium salts to oxidising conditions were unsuccessful even at elevated pH. These data are consistent with rhodocene being highly unstable and may indicate that iridocene is even more unstable still.<a class="footnote-ref" id="fnref:55" href="#fn:55">55</a>

<h2 id="speciation">Speciation</h2>
The <a href="/facts/18-electron_rule/KNfG6tpM">18-electron rule</a> is the equivalent of the <a href="/facts/Octet_rule/X9jxSsos">octet rule</a> in <a href="/facts/Main_group/2AG3QxJl">main group</a> chemistry and provides a useful guide for predicting the stability of <a href="/facts/Organometallic_compound/bhqFz2P0">organometallic compounds</a>.<a class="footnote-ref" id="fnref:56" href="#fn:56">56</a> It predicts that organometallic species "in which the sum of the metal valence electrons plus the electrons donated by the ligand groups total 18 are likely to be stable."<a class="footnote-ref" id="fnref:57" href="#fn:57">57</a> This helps to explain the unusually high stability observed for ferrocene<a class="footnote-ref" id="fnref:58" href="#fn:58">58</a> and for the cobalticinium and rhodocenium cations<a class="footnote-ref" id="fnref:59" href="#fn:59">59</a> – all three species have <a href="/facts/Analog_(chemistry)/3aysWSVG">analogous</a> geometries and are <a href="/facts/Isoelectronic/SbzuaI05">isoelectronic</a> 18-valence electron structures. The instability of rhodocene and cobaltocene is also understandable in terms of the 18-electron rule, in that both are 19-valence electron structures; this explains early difficulties in isolating rhodocene from rhodocenium solutions.<a class="footnote-ref" id="fnref:60" href="#fn:60">60</a> The chemistry of rhodocene is dominated by the drive to attain an 18-electron configuration.<a class="footnote-ref" id="fnref:61" href="#fn:61">61</a>
Rhodocene exists as [Rh(C5H5)2], a <a href="/facts/Paramagnetic/vj1SMbbp">paramagnetic</a> 19-valence electron <a href="/facts/Radical_(chemistry)/qFymuVoT">radical</a> <a href="/facts/Monomer/Gw3CltMb">monomer</a> only at or below −196 °C (−320.8 °F) (<a href="/facts/Liquid_nitrogen/IGWI1h00">liquid nitrogen</a> temperatures) or above 150 °C (302 °F) in the <a href="/facts/Gas_phase/NkHXDdN3">gas phase</a>.<a class="footnote-ref" id="fnref:62" href="#fn:62">62</a><a class="footnote-ref" id="fnref:63" href="#fn:63">63</a><a class="footnote-ref" id="fnref:64" href="#fn:64">64</a> It is this monomeric form that displays the typical <a href="/facts/Staggered_conformation/EYoNI8Pv">staggered</a> <a href="/facts/Metallocene/AecA5Qrk">metallocene</a> sandwich structure. At room temperature (25 °C [77 °F]), the lifetime of the monomeric form in <a href="/facts/Acetonitrile/fNpbvRp6">acetonitrile</a> is less than two seconds;<a class="footnote-ref" id="fnref:65" href="#fn:65">65</a> and rhodocene forms [Rh(C5H5)2]2, a <a href="/facts/Diamagnetic/dgFavn9K">diamagnetic</a> 18-valence electron <a href="/facts/Bridge_(chemical)/WfstMqT6">bridged</a> <a href="/facts/Dimer_(chemistry)/mU64PnEq">dimeric</a> ansa-metallocene structure.<a class="footnote-ref" id="fnref:66" href="#fn:66">66</a> <a href="/facts/Electron_spin_resonance/TDHV8c9e">Electron spin resonance</a> (ESR), <a href="/facts/Nuclear_magnetic_resonance/pGYAn9Bz">nuclear magnetic resonance</a> (NMR) and <a href="/facts/Infrared_spectroscopy/u1Hzx617">infrared spectroscopic</a> (IR) measurements point to the presence of an <a href="/facts/Chemical_equilibrium/y8ToYXHp">equilibrium</a> interconverting the monomeric and dimeric forms.<a class="footnote-ref" id="fnref:67" href="#fn:67">67</a> ESR evidence confirms that the monomer possesses a high order <a href="/facts/Axis_of_symmetry/bpvJqcnS">axis of symmetry</a> (Cn, n > 2) with a <a href="/facts/Mirror_plane/5JHWnrql">mirror plane</a> (σ) perpendicular to it as <a href="/facts/Molecular_symmetry/BrDxAkTK">symmetry elements</a>; this experimentally demonstrates that the monomer does possess the typical sandwich structure of a metallocene<a class="footnote-ref" id="fnref:68" href="#fn:68">68</a><a class="footnote-ref" id="fnref:69" href="#fn:69">69</a> although the interpretation of the ESR data has been questioned.<a class="footnote-ref" id="fnref:70" href="#fn:70">70</a> The decomposition pathway of the monomer has also been studied by <a href="/facts/Mass_spectrometry/RH5WBHJ2">mass spectrometry</a>.<a class="footnote-ref" id="fnref:71" href="#fn:71">71</a> The dimerisation is a <a href="/facts/Redox/Pyd5RVcj">redox</a> process; the dimer is a rhodium(I) species and the monomer has a rhodium(II) centre.<a class="footnote-ref" id="fnref:72" href="#fn:72">72</a> <a href="/facts/Rhodium/xmBd9aFG">Rhodium</a> typically occupies <a href="/facts/Oxidation_state/W53W6s6V">oxidation states</a> +I or +III in its stable compounds.<a class="footnote-ref" id="fnref:73" href="#fn:73">73</a>
This dimerisation process has the overall effect of decreasing the <a href="/facts/Electron_counting/1bPUvehY">electron count</a> around the rhodium centre from 19 to 18. This occurs because the <a href="/facts/Oxidative_coupling/4H7Tjxf0">oxidative coupling</a> of the two cyclopentadienyl ligands produces a new ligand with lower <a href="/facts/Hapticity/7MDtzehL">hapticity</a> and which donates fewer electrons to the metal centre. The term hapticity is used to indicate the "number of carbon (or other) atoms through which [a ligand] binds (n)"<a class="footnote-ref" id="fnref:74" href="#fn:74">74</a> to a metal centre and is symbolised as ηn. For example, the ethylene ligand in Zeise's salt is bound to the platinum centre through both carbon atoms, and it hence formally has the formula K[PtCl3(η2-C2H4)]·H2O.<a class="footnote-ref" id="fnref:75" href="#fn:75">75</a> The carbonyl ligands in nickel tetracarbonyl are each bound through only a carbon atom and are hence described as monohapto ligands, but η1-notations are typically omitted in formulae. The cyclopentadienyl ligands in many <a href="/facts/Metallocene/AecA5Qrk">metallocene</a> and <a href="/facts/Sandwich_compound/uhupMGDU">half-sandwich compounds</a> are pentahapto ligands, hence the formula [Rh(η5-C5H5)2] for the rhodocene monomer. In the rhodocene dimer, the coupled cyclopentadienyl ligands are 4-electron tetrahapto donors to each rhodium(I) metal centre, in contrast to the 6-electron<a class="footnote-ref" id="fnref:76" href="#fn:76">76</a> pentahapto cyclopentadienyl donors. The increased stability of the 18-valence electron rhodium(I) dimer species as compared to the 19-valence electron rhodium(II) monomer likely explains why the monomer is only detected under extreme conditions.<a class="footnote-ref" id="fnref:77" href="#fn:77">77</a><a class="footnote-ref" id="fnref:78" href="#fn:78">78</a>

Cotton and Wilkinson demonstrated<a class="footnote-ref" id="fnref:79" href="#fn:79">79</a> that the 18-valence electron rhodium(III) rhodocenium cation [Rh(η5-C5H5)2]+ can be reduced in aqueous solution to the monomeric form; they were unable to isolate the neutral product as not only can it dimerise, the rhodium(II) radical monomer can also spontaneously form the mixed-hapticity stable rhodium(I) species [(η5-C5H5)Rh(η4-C5H6)].<a class="footnote-ref" id="fnref:80" href="#fn:80">80</a> The differences between rhodocene and this derivative are found in two areas:

<ol><li>One of the bound cyclopentadienyl ligands has formally gained a hydrogen atom to become cyclopentadiene, which remains bound to the metal centre but now as a 4-electron η4- donor.</li>
<li>The rhodium(II) metal centre has been reduced to rhodium(I).</li></ol>
These two changes make the derivative an 18-valence electron species. Fischer and colleagues hypothesised that the formation of this rhodocene derivative might occur in separate protonation and reduction steps, but published no evidence to support this suggestion.<a class="footnote-ref" id="fnref:81" href="#fn:81">81</a> (η4-Cyclopentadiene)(η5-cyclopentadienyl)rhodium(I), the resulting compound, is an unusual organometallic complex in that it has both a cyclopentadienyl anion and cyclopentadiene itself as ligands. It has been shown that this compound can also be prepared by <a href="/facts/Sodium_borohydride/D2Hwts5x">sodium borohydride</a> reduction of a rhodocenium solution in aqueous <a href="/facts/Ethanol/3lCLfBSP">ethanol</a>; the researchers who made this discovery characterised the product as biscyclopentadienylrhodium hydride.<a class="footnote-ref" id="fnref:82" href="#fn:82">82</a>
Fischer and co-workers also studied the chemistry of iridocene, the third transition series analogue of rhodocene and cobaltocene, finding the chemistry of rhodocene and iridocene are generally similar. The synthesis of numerous iridocenium salts including the tribromide and <a href="/facts/Hexafluorophosphate/mXYXC2Bf">hexafluorophosphate</a> have been described.<a class="footnote-ref" id="fnref:83" href="#fn:83">83</a> Just as with rhodocene, iridocene dimerises at room temperature but a monomer form can be detected at low temperatures and in gas phase and IR, NMR, and ESR measurements indicate a chemical equilibrium is present and confirm the sandwich structure of the iridocene monomer.<a class="footnote-ref" id="fnref:84" href="#fn:84">84</a><a class="footnote-ref" id="fnref:85" href="#fn:85">85</a> The complex [(η5-C5H5)Ir(η4-C5H6)], the analogue of rhodocene derivative reported by Fischer,<a class="footnote-ref" id="fnref:86" href="#fn:86">86</a> has also been studied and demonstrates properties consistent with a greater degree of π-backbonding in iridium(I) systems than is found in the analogous cobalt(I) or rhodium(I) cases.<a class="footnote-ref" id="fnref:87" href="#fn:87">87</a>

<h2 id="synthesis">Synthesis</h2>
Rhodocenium salts were first reported<a class="footnote-ref" id="fnref:88" href="#fn:88">88</a> within two years of the discovery of ferrocene.<a class="footnote-ref" id="fnref:89" href="#fn:89">89</a> These salts were prepared by reacting the <a href="/facts/Carbanion/AyYH2udQ">carbanion</a> <a href="/facts/Grignard_reaction/DfhXQ6SG">Grignard reagent</a> cyclopentadienylmagnesium bromide (C5H5MgBr) with <a href="/facts/Metal_acetylacetonates/xfs1jkzy">tris(acetylacetonato)rhodium(III)</a> (Rh(acac)3). More recently, gas-phase rhodocenium cations have been generated by a <a href="/facts/Redox/Pyd5RVcj">redox</a> <a href="/facts/Transmetalation/Mm0hQzp3">transmetalation</a> reaction of rhodium(I) ions with ferrocene or <a href="/facts/Nickelocene/MpqA1LIF">nickelocene</a>.<a class="footnote-ref" id="fnref:90" href="#fn:90">90</a>

Rh+ + [(η5-C5H5)2M] → M + [(η5-C5H5)2Rh]+     M = Ni or Fe
Modern <a href="/facts/Microwave_chemistry/nTcqokMz">microwave synthetic methods</a> have also been reported.<a class="footnote-ref" id="fnref:91" href="#fn:91">91</a> Rhodocenium hexafluorophosphate forms after reaction of cyclopentadiene and <a href="/facts/Rhodium(III)_chloride/ownYu50L">rhodium(III) chloride hydrate</a> in <a href="/facts/Methanol/EIfpoNCg">methanol</a> following <a href="/facts/Work-up_(chemistry)/LURlLf4C">work-up</a> with methanolic <a href="/facts/Ammonium_hexafluorophosphate/Ig7CFPVf">ammonium hexafluorophosphate</a>; the reaction <a href="/facts/Yield_(chemistry)/fkBhd9dH">yield</a> exceeds 60% with only 30 seconds of exposure to <a href="/facts/Microwave_radiation/HmVSN31Y">microwave radiation</a>.<a class="footnote-ref" id="fnref:92" href="#fn:92">92</a>

RhCl3·xH2O + 2 C5H6 + NH4PF6 → [(η5-C5H5)2PF6↓ + 2 HCl + NH4Cl + xH2O
Rhodocene itself is then formed by reduction of rhodocenium salts with molten <a href="/facts/Sodium/eYuSPu2V">sodium</a>.<a class="footnote-ref" id="fnref:93" href="#fn:93">93</a> If a rhodocenium containing melt is treated with sodium or potassium metals and then <a href="/facts/Sublimation_(phase_transition)/KDoKokMn">sublimed</a> onto a liquid nitrogen-cooled cold finger, a black polycrystalline material results.<a class="footnote-ref" id="fnref:94" href="#fn:94">94</a> Warming this material to room temperature produces a yellow solid which has been confirmed as the rhodocene dimer. A similar method can be used to prepare the iridocene dimer.<a class="footnote-ref" id="fnref:95" href="#fn:95">95</a>

<h2 id="substituted-rhodocenes-and-rhodocenium-salts">Substituted rhodocenes and rhodocenium salts</h2>
<h3>The [(η5-C5tBu3H2)Rh(η5-C5H5)]+ cation</h3>
Novel approaches to synthesising substituted cyclopentadienyl complexes have been developed using substituted vinylcyclopropene starting materials.<a class="footnote-ref" id="fnref:96" href="#fn:96">96</a><a class="footnote-ref" id="fnref:97" href="#fn:97">97</a><a class="footnote-ref" id="fnref:98" href="#fn:98">98</a> Ring-enlarging <a href="/facts/Vinylcyclopropane_rearrangement/QfHxtHfz">vinylcyclopropane rearrangement</a> reactions to produce cyclopentenes are well known<a class="footnote-ref" id="fnref:99" href="#fn:99">99</a> and serve as precedent for vinylcyclopropenes <a href="/facts/Rearrangement_reaction/B1XTteIJ">rearranging</a> to cyclopentadienes. The [(η5-C5<a href="/facts/Tert-butyl/fbTF0UkI">tBu</a>3H2)Rh(η5-C5H5)]+ cation has been generated by a reaction sequence beginning with addition of the chlorobisethylenerhodium(I) dimer, [(η2-C2H4)2Rh(μ-Cl)]2, to 1,2,3-tri-tert-butyl-3-vinyl-1-cyclopropene followed by reaction with <a href="/facts/Thallium_cyclopentadienide/h9D46ftP">thallium cyclopentadienide</a>:<a class="footnote-ref" id="fnref:100" href="#fn:100">100</a><a class="footnote-ref" id="fnref:101" href="#fn:101">101</a>

The 18-valence electron rhodium(III) pentadienediyl species generated by this reaction demonstrates again the instability of the rhodocene moiety, in that it can be refluxed in toluene for months without 1,2,3-tri-tert-butylrhodocene forming but in oxidising conditions the 1,2,3-tri-tert-butylrhodocenium cation forms rapidly.<a class="footnote-ref" id="fnref:102" href="#fn:102">102</a> Cyclic voltammetry has been used to investigate this and similar processes in detail.<a class="footnote-ref" id="fnref:103" href="#fn:103">103</a><a class="footnote-ref" id="fnref:104" href="#fn:104">104</a> The mechanism of the reaction has been shown to involve a loss of one electron from the pentadienediyl ligand followed by a fast rearrangement (with loss of a hydrogen atom) to form the 1,2,3-tri-tert-butylrhodocenium cation.<a class="footnote-ref" id="fnref:105" href="#fn:105">105</a> Both the <a href="/facts/Tetrafluoroborate/46EEToo6">tetrafluoroborate</a> and hexafluorophosphate salts of this cation have been structurally characterised by X-ray crystallography.<a class="footnote-ref" id="fnref:106" href="#fn:106">106</a>
[(η5-C5tBu3H2)Rh(η5-C5H5)]BF4 forms a colourless <a href="/facts/Centrosymmetric/nIFJqujU">centrosymmetric</a> <a href="/facts/Monoclinic_crystal_system/5PCwXvzX">monoclinic</a> crystal belonging to the P21/c <a href="/facts/Space_group/2XQx0J7M">space group</a>, and with a <a href="/facts/Density/92p6hh9I">density</a> of 1.486 g cm−3.<a class="footnote-ref" id="fnref:107" href="#fn:107">107</a> Looking at the <a href="/facts/Molecular_graphics/rLuIm8hP">ORTEP</a> diagram of the structure of the cation (at right), it is evident that it possesses the typical geometry expected of a rhodocene or rhodocenium cation. The two cyclopentadienyl rings are close to parallel (the <a href="/facts/Centroid/H6dDbg0h">centroid</a>–Rh–centroid angle is 177.2°) and the rhodium centre is slightly closer to the substituted cyclopentadienyl ring (Rh–centroid distances are 1.819 <a href="/facts/%25C3%2585ngstr%25C3%25B6m/T5mjdd97">Å</a> and 1.795 Å), an observation attributed to the greater inductive effect of the <a href="/facts/Tert-butyl/fbTF0UkI">tert-butyl</a> groups on the substituted ligand.<a class="footnote-ref" id="fnref:108" href="#fn:108">108</a> The ORTEP diagram shows that the cation adopts an eclipsed conformation in the solid state. The crystal structure of the hexafluorophosphate salt shows three crystallographically independent cations, one eclipsed, one staggered, and one which is rotationally disordered.<a class="footnote-ref" id="fnref:109" href="#fn:109">109</a> This suggests that the conformation adopted is dependent on the anion present and also that the energy barrier to rotation is low – in ferrocene, the rotational energy barrier is known to be ~5 kJ mol−1 in both solution and gas phase.<a class="footnote-ref" id="fnref:110" href="#fn:110">110</a>

The diagram above shows the rhodium–carbon (in red, inside pentagons on the left) and carbon–carbon (in blue, outside pentagons on the left) bond distances for both ligands, along with the bond angles (in green, inside pentagons on the right) within each cyclopentadienyl ring. The atom labels used are the same as those shown in the crystal structure above. Within the unsubstituted cyclopentadienyl ligand, the carbon–carbon bond lengths vary between 1.35 Å and 1.40 Å and the internal bond angles vary between 107° and 109°. For comparison, the internal angle at each vertex of a <a href="/facts/Regular_pentagon/xoq5oJeQ">regular pentagon</a> is 108°. The rhodium–carbon bond lengths vary between 2.16 Å and 2.18 Å.<a class="footnote-ref" id="fnref:111" href="#fn:111">111</a> These results are consistent with η5-coordination of the ligand to the metal centre. In the case of the substituted cyclopentadienyl ligand, there is somewhat greater variation: carbon–carbon bond lengths vary between 1.39 Å and 1.48 Å, the internal bond angles vary between 106° and 111°, and the rhodium–carbon bond lengths vary between 2.14 Å and 2.20 Å. The greater variation in the substituted ligand is attributed to the distortions necessary to relieve the steric strain imposed by neighbouring tert-butyl substituents; despite these variations, the data demonstrate that the substituted cyclopentadienyl is also η5-coordinated.<a class="footnote-ref" id="fnref:112" href="#fn:112">112</a>
The stability of metallocenes changes with ring substitution. Comparing the reduction potentials of the cobaltocenium and decamethylcobaltocenium cations shows that the decamethyl species is ca. 600 mV more reducing than its parent metallocene,<a class="footnote-ref" id="fnref:113" href="#fn:113">113</a> a situation also observed in the ferrocene<a class="footnote-ref" id="fnref:114" href="#fn:114">114</a> and rhodocene systems.<a class="footnote-ref" id="fnref:115" href="#fn:115">115</a> The following data are presented relative to the ferrocenium / ferrocene <a href="/facts/Redox_couple/Pyd5RVcj">redox couple</a>:<a class="footnote-ref" id="fnref:116" href="#fn:116">116</a>

<table><tbody><tr><th><a href="/facts/Half-reaction/4pDKxYa6">Half-reaction</a></th><th>E° (V)</th></tr><tr><td>[Fe(C5H5)2]+ + e− ⇌ [Fe(C5H5)2]</td><td>0 (by definition)</td></tr><tr><td>[Fe(C5Me5)2]+ + e− ⇌ [Fe(C5Me5)2]</td><td>−0.59<a class="footnote-ref" id="fnref:117" href="#fn:117">117</a></td></tr><tr><td>[Co(C5H5)2]+ + e− ⇌ [Co(C5H5)2]</td><td>−1.33<a class="footnote-ref" id="fnref:118" href="#fn:118">118</a></td></tr><tr><td>[Co(C5Me5)2]+ + e− ⇌ [Co(C5Me5)2]</td><td>−1.94<a class="footnote-ref" id="fnref:119" href="#fn:119">119</a></td></tr><tr><td>[Rh(C5H5)2]+ + e− ⇌ [Rh(C5H5)2]</td><td>−1.79<a class="footnote-ref" id="fnref:120" href="#fn:120">120</a> †</td></tr><tr><td>[Rh(C5Me5)2]+ + e− ⇌ [Rh(C5Me5)2]</td><td>−2.38<a class="footnote-ref" id="fnref:121" href="#fn:121">121</a></td></tr><tr><td>[(C5<a href="/facts/Tert-butyl/fbTF0UkI">tBu</a>3H2)Rh(C5H5)]+ + e− ⇌ [(C5<a href="/facts/Tert-butyl/fbTF0UkI">tBu</a>3H2)Rh(C5H5)]</td><td>−1.83<a class="footnote-ref" id="fnref:122" href="#fn:122">122</a></td></tr><tr><td>[(C5tBu3H2)Rh(C5Me5)]+ + e− ⇌ [(C5tBu3H2)Rh(C5Me5)]</td><td>−2.03 <a class="footnote-ref" id="fnref:123" href="#fn:123">123</a></td></tr><tr><td>[(C5H5Ir(C5Me5)]+ + e− ⇌ [(C5H5Ir(C5Me5)]</td><td>−2.41<a class="footnote-ref" id="fnref:124" href="#fn:124">124</a> †</td></tr><tr><td>[Ir(C5Me5)2]+ + e− ⇌ [Ir(C5Me5)2]</td><td>−2.65<a class="footnote-ref" id="fnref:125" href="#fn:125">125</a> †</td></tr><tr><td>† after correcting by 0.38 V<a class="footnote-ref" id="fnref:126" href="#fn:126">126</a> for the different standard</td></tr></tbody></table>
The differences in reduction potentials are attributed in the cobaltocenium system to the inductive effect of the alkyl groups,<a class="footnote-ref" id="fnref:127" href="#fn:127">127</a> further stabilising the 18-valence electron species. A similar effect is seen in the rhodocenium data shown above, again consistent with inductive effects.<a class="footnote-ref" id="fnref:128" href="#fn:128">128</a> In the substituted iridocenium system, cyclic voltammetry investigations shows irreversible reductions at temperatures as low as −60 °C;<a class="footnote-ref" id="fnref:129" href="#fn:129">129</a> by comparison, the reduction of the corresponding rhodocenes is quasi-reversible at room temperature and fully reversible at −35 °C.<a class="footnote-ref" id="fnref:130" href="#fn:130">130</a> The irreversibility of the substituted iridocenium reductions is attributed to the extremely rapid dimerisation of the resulting 19-valence electron species, which further illustrates that iridocenes are less stable than their corresponding rhodocenes.<a class="footnote-ref" id="fnref:131" href="#fn:131">131</a>

<h3>Pentasubstituted cyclopentadienyl ligands</h3>
The body of knowledge concerning compounds with penta-substituted cyclopentadienyl ligands is extensive, with organometallic <a href="/facts/Complex_(chemistry)/1tCyeQUm">complexes</a> of the <a href="/facts/Pentamethylcyclopentadiene/EubVXcaF">pentamethylcyclopentadienyl</a> and pentaphenylcyclopentadienyl ligands being well-known.<a class="footnote-ref" id="fnref:132" href="#fn:132">132</a> Substitutions on the cyclopentadienyl rings of rhodocenes and rhodocenium salts produce compounds of higher stability as they allow for the increased delocalisation of positive charge or <a href="/facts/Electron_density/L5yDsk6j">electron density</a> and also provide <a href="/facts/Steric_hindrance/TnX5c8HG">steric hindrance</a> against other species approaching the metal centre.<a class="footnote-ref" id="fnref:133" href="#fn:133">133</a> Various mono- and di-substituted rhodocenium species are known, but substantial stabilisation is not achieved without greater substitutions.<a class="footnote-ref" id="fnref:134" href="#fn:134">134</a> Known highly substituted rhodocenium salts include decamethylrhodocenium hexafluorophosphate [(η5-C5<a href="/facts/Methyl/fkM97EMD">Me</a>5)2Rh]PF6,<a class="footnote-ref" id="fnref:135" href="#fn:135">135</a> decaisopropylrhodocenium hexafluorophosphate [(η5-C5<a href="/facts/Isopropyl/fl8SMScE">iPr</a>5)2Rh]PF6,<a class="footnote-ref" id="fnref:136" href="#fn:136">136</a> and octaphenylrhodocenium hexafluorophosphate [(η5-C5<a href="/facts/Phenyl/Ql9Yx5uu">Ph</a>4H)2Rh]PF6.<a class="footnote-ref" id="fnref:137" href="#fn:137">137</a><a class="footnote-ref" id="fnref:138" href="#fn:138">138</a> Decamethylrhodocenium tetrafluoroborate can be synthesised from the tris(acetone) complex [(η5-C5Me5)Rh(Me2CO)3](BF4)2 by reaction with <a href="/facts/Pentamethylcyclopentadiene/EubVXcaF">pentamethylcyclopentadiene</a>, and the analogous iridium synthesis is also known.<a class="footnote-ref" id="fnref:139" href="#fn:139">139</a> Decaisopropylrhodicnium hexafluorophosphate was synthesised in <a href="/facts/1%2c2-dimethoxyethane/2AdJp7sr">1,2-dimethoxyethane</a> (<a href="/facts/Solvent/LoMPMz8S">solvent</a>) in an unusual <a href="/facts/One-pot_synthesis/6CgQGt4D">one-pot synthesis</a> that involves the formation of 20 <a href="/facts/Carbon-carbon_bond/nLE4z5BK">carbon–carbon bonds</a>:<a class="footnote-ref" id="fnref:140" href="#fn:140">140</a>

In a similar reaction, pentaisopropylrhodocenium hexafluorophosphate [(η5-C5iPr5)Rh(η5-C5H5)]PF6 can be synthesised from pentamethylrhodocenium hexafluorophosphate [(η5-C5Me5)Rh(η5-C5H5)]PF6 in 80% yield.<a class="footnote-ref" id="fnref:141" href="#fn:141">141</a> These reactions demonstrate that the acidity of the methyl hydrogens in a pentamethylcyclopentadienyl complex can be considerably increased by the presence of the metal centre. Mechanistically, the reaction proceeds with <a href="/facts/Potassium_hydroxide/NBhPBuXc">potassium hydroxide</a> deprotonating a methyl group and the resulting carbanion undergoing <a href="/facts/Nucleophilic_substitution/MLLeSsnr">nucleophilic substitution</a> with <a href="/facts/Methyl_iodide/uUKdue80">methyl iodide</a> to form a new carbon–carbon bond.<a class="footnote-ref" id="fnref:142" href="#fn:142">142</a>
The compounds pentaphenylrhodocenium <a href="/facts/Tetrafluoroborate/46EEToo6">tetrafluoroborate</a> [(η5-C5Ph5)Rh(η5-C5H5)]BF4, and pentamethylpentaphenylrhodocenium tetrafluoroborate [(η5-C5Ph5)Rh(η5-C5Me5)]BF4 have also been reported. They demonstrate that rhodium sandwich compounds can be prepared from half-sandwich precursors. For example, in an approach broadly similar to the tris(acetone) synthesis of decamethylrhodocenium tetrafluoroborate,<a class="footnote-ref" id="fnref:143" href="#fn:143">143</a> pentaphenylrhodocenium tetrafluoroborate has been synthesised from the tris(<a href="/facts/Acetonitrile/fNpbvRp6">acetonitrile</a>) salt [(η5-C5Ph5)Rh(CH3CN)3](BF4)2 by reaction with <a href="/facts/Sodium_cyclopentadienide/l7JaX1NP">sodium cyclopentadienide</a>:<a class="footnote-ref" id="fnref:144" href="#fn:144">144</a>

[(η5-C5Ph5)Rh(MeCN)3](BF4)2 + NaC5H5 → [(η5-C5Ph5)Rh(η5-C5H5)]BF4 + NaBF4 + 3 MeCN

Octaphenylrhodocene, [(η5-C5Ph4H)2Rh], is the first rhodocene derivative to be isolated at room temperature. Its olive-green crystals decompose rapidly in solution, and within minutes in air, demonstrating a dramatically greater air sensitivity than the analogous <a href="/facts/Cobalt/gqhScEw4">cobalt</a> complex, although it is significantly more stable than rhodocene itself. This difference is attributed to the relatively lower stability of the rhodium(II) state as compared to the cobalt(II) state.<a class="footnote-ref" id="fnref:145" href="#fn:145">145</a><a class="footnote-ref" id="fnref:146" href="#fn:146">146</a> The reduction potential for the [(η5-C5Ph4H)2Rh]+ cation (measured in <a href="/facts/Dimethylformamide/wjI8UfB4">dimethylformamide</a> relative the ferrocenium / ferrocene couple) is −1.44 V, consistent with the greater thermodynamic stabilisation of the rhodocene by the C5HPh4 ligand compared with the C5H5 or C5Me5 ligands.<a class="footnote-ref" id="fnref:147" href="#fn:147">147</a> Cobaltocene is a useful one-electron <a href="/facts/Reducing_agent/CtfSGVXC">reducing agent</a> in the research laboratory as it is soluble in <a href="/facts/Non-polar/KlIJ0zjB">non-polar</a> organic solvents,<a class="footnote-ref" id="fnref:148" href="#fn:148">148</a> and its redox couple is sufficiently well behaved that it may be used as an <a href="/facts/Internal_standard/YdqEc1vq">internal standard</a> in <a href="/facts/Cyclic_voltammetry/fpTNzaEB">cyclic voltammetry</a>.<a class="footnote-ref" id="fnref:149" href="#fn:149">149</a> No substituted rhodocene yet prepared has demonstrated sufficient stability to be used in a similar way.
The synthesis of octaphenylrhodocene proceeds in three steps, with a <a href="/facts/Diglyme/GEAdmlEX">diglyme</a> reflux followed by workup with <a href="/facts/Hexafluorophosphoric_acid/wWSB2sm7">hexafluorophosphoric acid</a>, then a <a href="/facts/Sodium_amalgam/NlTRR9eq">sodium amalgam</a> reduction in <a href="/facts/Tetrahydrofuran/P5t6SkMK">tetrahydrofuran</a>:<a class="footnote-ref" id="fnref:150" href="#fn:150">150</a>

Rh(acac)3 + 2 KC5Ph4H → [(η5-C5Ph4H)2Rh]+ + 2 K+ + 3 acac−
[(η5-C5Ph4H)2Rh]+ + 3 acac− + 3 HPF6 → [(η5-C5Ph4H)2Rh]PF6 + 3 <a href="/facts/Acetylacetone/Ku1k97xz">Hacac</a> + 2 PF6−
[(η5-C5Ph4H)2Rh]PF6 + Na/Hg → [(η5-C5Ph4H)2Rh] + NaPF6
The <a href="/facts/X-ray_crystallography/WtI0F5DN">crystal structure</a> of octaphenylrhodocene shows a staggered conformation<a class="footnote-ref" id="fnref:151" href="#fn:151">151</a> (similar to that of ferrocene, and in contrast to the <a href="/facts/Eclipsed_conformation/krQjD1Ll">eclipsed conformation</a> of <a href="/facts/Ruthenocene/1yYGoc0y">ruthenocene</a>).<a class="footnote-ref" id="fnref:152" href="#fn:152">152</a> The rhodium–centroid distance is 1.904 Å and the rhodium–carbon bond lengths average 2.26 Å; the carbon–carbon bond lengths average 1.44 Å.<a class="footnote-ref" id="fnref:153" href="#fn:153">153</a> These distances are all similar to those found in the 1,2,3-tri-tert-butylrhodocenium cation described above, with the one difference that the effective size of the rhodium centre appears larger, an observation consistent with the expanded ionic radius of rhodium(II) compared with rhodium(III).<a class="footnote-ref" id="fnref:154" href="#fn:154">154</a>

<h2 id="applications">Applications</h2>
<h3>Biomedical use of a derivative</h3>

There has been extensive research into <a href="/facts/Metallopharmaceutical/9K1rKSp2">metallopharmaceuticals</a>,<a class="footnote-ref" id="fnref:155" href="#fn:155">155</a><a class="footnote-ref" id="fnref:156" href="#fn:156">156</a> including discussion of rhodium compounds in medicine.<a class="footnote-ref" id="fnref:157" href="#fn:157">157</a> A substantial body of research has examined using metallocene derivatives of <a href="/facts/Ruthenium/uzCm4ufn">ruthenium</a><a class="footnote-ref" id="fnref:158" href="#fn:158">158</a> and iron<a class="footnote-ref" id="fnref:159" href="#fn:159">159</a> as metallopharmaceuticals. One area of such research has utilised metallocenes in place of the fluorophenyl group in <a href="/facts/Haloperidol/ue4TJKmu">haloperidol</a>,<a class="footnote-ref" id="fnref:160" href="#fn:160">160</a> which is a <a href="/facts/Pharmaceutical/h9mhwWP2">pharmaceutical</a> classified as a <a href="/facts/Typical_antipsychotic/TpHx2fqp">typical antipsychotic</a>. The ferrocenyl–haloperidol compound investigated has structure (C5H5)Fe(C5H4)–C(=O)–(CH2)3–N(CH2CH2)2C(OH)–C6H4Cl and can be converted to the ruthenium analog via a transmetalation reaction. Using the <a href="/facts/Radioactive/Ya6FVjt4">radioactive</a> <a href="/facts/Isotope/KD9B1y6o">isotope</a> <a href="/facts/Ruthenium-103/C9GGqqPM">103Ru</a> produces a ruthenocenyl–haloperidol radiopharmaceutical with a high affinity for <a href="/facts/Lung/gTWXI6Fj">lung</a> but not <a href="/facts/Brain/EKla9NkJ">brain</a> <a href="/facts/Tissue_(biology)/H7CpML7A">tissue</a> in <a href="/facts/Mice/6m1X7m0g">mice</a> and <a href="/facts/Rat/5rn6uEKM">rats</a>.<a class="footnote-ref" id="fnref:161" href="#fn:161">161</a> <a href="/facts/Beta-decay/vxTBlckp">Beta-decay</a> of 103Ru produces the <a href="/facts/Nuclear_isomer/L8DHcqnk">metastable isotope</a> <a href="/facts/Rhodium-103m/fYAb2dz8">103mRh</a> in a rhodocenyl–haloperidol compound. This compound, like other rhodocene derivatives, has an unstable 19-valence electron configuration and rapidly oxidises to the expected cationic rhodocenium–haloperidol species.<a class="footnote-ref" id="fnref:162" href="#fn:162">162</a> The separation of the ruthenocenyl–haloperidol and the rhodocenium–haloperidol species and the distributions of each amongst bodily organs has been studied.<a class="footnote-ref" id="fnref:163" href="#fn:163">163</a> 103mRh has a <a href="/facts/Half-life/kGVH8BAB">half-life</a> of 56 min and emits a <a href="/facts/Gamma_ray/5Drf913J">gamma ray</a> of energy 39.8 <a href="/facts/KeV/YMT1fqX9">keV</a>, so the <a href="/facts/Gamma-decay/5Drf913J">gamma-decay</a> of the rhodium isotope should follow soon after the beta-decay of the ruthenium isotope. Beta- and gamma-emitting <a href="/facts/Radionuclide/5WpCfbdq">radionuclides</a> used medically include <a href="/facts/Iodine-131/cMvRbKfK">131I</a>, <a href="/facts/Radiopharmacology/EArnJn3N">59Fe</a>, and <a href="/facts/Radiopharmacology/EArnJn3N">47Ca</a>, and 103mRh has been proposed for use in <a href="/facts/Radiotherapy/j60q5EC6">radiotherapy</a> for small tumours.<a class="footnote-ref" id="fnref:164" href="#fn:164">164</a>

<h3>Metal–metal interactions in linked metallocenes</h3>

The original motivation for research investigations of the rhodocene system was to understand the nature of and bonding within the metallocene class of compounds. In more recent times, interest has been rekindled by the desire to explore and apply the metal–metal interactions that occur when metallocene systems are linked.<a class="footnote-ref" id="fnref:165" href="#fn:165">165</a> Potential applications for such systems include <a href="/facts/Molecular_electronics/nC056GHG">molecular electronics</a>,<a class="footnote-ref" id="fnref:166" href="#fn:166">166</a> semi-conducting (and possibly <a href="/facts/Ferromagnetic/fH3HFnSl">ferromagnetic</a>) metallocene polymers (an example of a <a href="/facts/Molecular_wire/E7dSB6mF">molecular wire</a>),<a class="footnote-ref" id="fnref:167" href="#fn:167">167</a> and exploring the threshold between <a href="/facts/Heterogeneous_catalysis/ju7E0bab">heterogeneous</a> and <a href="/facts/Homogeneous_catalysis/c7wplnb2">homogeneous catalysis</a>.<a class="footnote-ref" id="fnref:168" href="#fn:168">168</a> Examples of known bimetallocenes and termetallocenes that possess the rhodocenyl moiety include the hexafluorophosphate salts of rhodocenylferrocene, 1,1'-dirhodocenylferrocene, and 1-cobaltocenyl-1'-rhodocenylferrocene,<a class="footnote-ref" id="fnref:169" href="#fn:169">169</a> each shown at right. Linked metallocenes can also be formed by introducing several metallocenyl substituents onto a single cyclopentadienyl ligand.<a class="footnote-ref" id="fnref:170" href="#fn:170">170</a>
Structural studies of termetallocene systems have shown they typically adopt an "eclipsed double transoid" "crankshaft" geometry.<a class="footnote-ref" id="fnref:171" href="#fn:171">171</a> Taking as an example the 1-cobaltocenyl-1'-rhodocenylferrocene cation shown above, this means that the cobaltocenyl and rhodocenyl moieties are eclipsed, and thus carbon atoms 1 and 1' on the central ferrocene core are as close to vertically aligned as is possible given the staggered conformation of the cyclopentadienyl rings within each metallocene unit. Viewed from side-on, this means termetallocenes resemble the down–up–down pattern of a <a href="/facts/Crankshaft/oYCe9nB3">crankshaft</a>.<a class="footnote-ref" id="fnref:172" href="#fn:172">172</a> The synthesis of this termetallocene involves the combining of rhodocenium and cobaltocenium solutions with <a href="/facts/Ferrocene/jKtu9Cp8">1,1'-dilithioferrocene</a>. This produces an uncharged intermediate with linked cyclopentadienyl–cyclopentadiene ligands whose bonding resembles that found in the rhodocene dimer. These ligands then react with the <a href="/facts/Triphenylmethyl_hexafluorophosphate/nH6vs45A">triphenylmethyl carbocation</a> to generate the termetallocene salt, [(η5-C5H5)Rh(μ-η5:η5-C5H4–C5H4)Fe(μ-η5:η5-C5H4–C5H4)Co(η5-C5H5)](PF6)2. This synthetic pathway is illustrated below:<a class="footnote-ref" id="fnref:173" href="#fn:173">173</a><a class="footnote-ref" id="fnref:174" href="#fn:174">174</a>

<h3>Rhodocenium-containing polymers</h3>
The first rhodocenium-containing side-chain polymers were prepared through controlled polymerization techniques such as <a href="/facts/Reversible_addition%25E2%2588%2592fragmentation_chain-transfer_polymerization/F3WPhFWK">reversible addition−fragmentation chain-transfer polymerization</a> (RAFT) and <a href="/facts/Ring-opening_metathesis_polymerisation/eJ8EUTJx">ring-opening metathesis polymerisation</a> (ROMP).<a class="footnote-ref" id="fnref:175" href="#fn:175">175</a>

<h2 id="notes">Notes</h2>

<h2 id="references">References</h2>

<ol>
<li id="fn:1">Crabtree, R. H. (2009). The Organometallic Chemistry of the Transition Metals (5th ed.). Hoboken, NJ: John Wiley and Sons. p. 2. ISBN 978-0-470-25762-3. An industrial application of transition metal organometallic chemistry appeared as early as the 1880s, when Ludwig Mond showed that nickel can be purified by using CO to pick up nickel in the form of gaseous Ni(CO)4 that can easily be separated from solid impurities and later be thermally decomposed to give pure nickel.... Recent work has shown the existence of a growing class of metalloenzymes having organometallic ligand environments – considered as the chemistry of metal ions having C-donor ligands such as CO or the methyl group <a href="978-0-470-25762-3" target="_blank">978-0-470-25762-3</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></li>
<li id="fn:2">El Murr, N.; Sheats, J. E.; Geiger, W. E.; Holloway, J. D. L. (1979). "Electrochemical Reduction Pathways of the Rhodocenium Ion. Dimerization and Reduction of Rhodocene". Inorganic Chemistry. 18 (6): 1443–1446. doi:10.1021/ic50196a007. <a href="/wiki/Inorganic_Chemistry_(journal)" target="_blank">/wiki/Inorganic_Chemistry_(journal)</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></li>
<li id="fn:3">Fischer, E. O.; Wawersik, H. (1966). "Über Aromatenkomplexe von Metallen. LXXXVIII. Über Monomeres und Dimeres Dicyclopentadienylrhodium und Dicyclopentadienyliridium und Über Ein Neues Verfahren Zur Darstellung Ungeladener Metall-Aromaten-Komplexe" [Aromatic Complexes of Metals. LXXXVIII. On the Monomers and Dimers Dicyclopentadienylrhodium and Dicyclopentadienyliridium and a New Method for the Preparation of Uncharged Metal-Aromatic Complexes]. Journal of Organometallic Chemistry (in German). 5 (6): 559–567. doi:10.1016/S0022-328X(00)85160-8. <a href="/wiki/Ernst_Otto_Fischer" target="_blank">/wiki/Ernst_Otto_Fischer</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></li>
<li id="fn:4">Keller, H. J.; Wawersik, H. (1967). "Spektroskopische Untersuchungen an Komplexverbindungen. VI. EPR-spektren von (C5H5)2Rh und (C5H5)2Ir" [Spectroscopic studies of complex compounds. VI. EPR spectra of (C5H5)2Rh and (C5H5)2Ir]. Journal of Organometallic Chemistry (in German). 8 (1): 185–188. doi:10.1016/S0022-328X(00)84718-X. <a href="/wiki/Journal_of_Organometallic_Chemistry" target="_blank">/wiki/Journal_of_Organometallic_Chemistry</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></li>
<li id="fn:5">Zeise, W. C. (1831). "Von der Wirkung zwischen Platinchlorid und Alkohol, und von den dabei entstehenden neuen Substanzen" [On the interaction between platinum chloride and alcohol, and the new substances thereby formed]. Annalen der Physik (in German). 97 (4): 497–541. Bibcode:1831AnP....97..497Z. doi:10.1002/andp.18310970402. Archived from the original on 6 August 2020. Retrieved 12 September 2020. <a href="/wiki/William_Christopher_Zeise" target="_blank">/wiki/William_Christopher_Zeise</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></li>
<li id="fn:6">Hunt, L. B. (1984). "The First Organometallic Compounds: William Christopher Zeise and his Platinum Complexes" (PDF). Platinum Metals Review. 28 (2): 76–83. Archived (PDF) from the original on 24 September 2015. Retrieved 8 January 2011. <a href="http://www.platinummetalsreview.com/pdf/pmr-v28-i2-076-083.pdf" target="_blank">http://www.platinummetalsreview.com/pdf/pmr-v28-i2-076-083.pdf</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></li>
<li id="fn:7">Winterton, N. (2002). "Some Notes on the Early Development of Models of Bonding in Olefin-Metal Complexes". In Leigh, G. J.; Winterton, N. (eds.). Modern Coordination Chemistry: The Legacy of Joseph Chatt. RSC Publishing. pp. 103–110. ISBN 9780854044696. Archived from the original on 26 January 2020. Retrieved 17 June 2017. <a href="9780854044696" target="_blank">9780854044696</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></li>
<li id="fn:8">Crabtree, R. H. (2009). The Organometallic Chemistry of the Transition Metals (5th ed.). Hoboken, NJ: John Wiley and Sons. p. 2. ISBN 978-0-470-25762-3. An industrial application of transition metal organometallic chemistry appeared as early as the 1880s, when Ludwig Mond showed that nickel can be purified by using CO to pick up nickel in the form of gaseous Ni(CO)4 that can easily be separated from solid impurities and later be thermally decomposed to give pure nickel.... Recent work has shown the existence of a growing class of metalloenzymes having organometallic ligand environments – considered as the chemistry of metal ions having C-donor ligands such as CO or the methyl group <a href="978-0-470-25762-3" target="_blank">978-0-470-25762-3</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></li>
<li id="fn:9">Laszlo, P.; Hoffmann, R. (2000). "Ferrocene: Ironclad History or Rashomon Tale?". Angewandte Chemie International Edition. 39 (1): 123–124. doi:10.1002/(SICI)1521-3773(20000103)39:1<123::AID-ANIE123>3.0.CO;2-Z. PMID 10649350. <a href="/wiki/Roald_Hoffman" target="_blank">/wiki/Roald_Hoffman</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></li>
<li id="fn:10">Federman Neto, A.; Pelegrino, A. C.; Darin, V. A. (2004). "Ferrocene: 50 Years of Transition Metal Organometallic Chemistry – From Organic and Inorganic to Supramolecular Chemistry". ChemInform. 35 (43). doi:10.1002/chin.200443242. (Abstract; original published in Trends in Organometallic Chemistry, 4:147–169, 2002) <a href="/wiki/ChemInform" target="_blank">/wiki/ChemInform</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></li>
<li id="fn:11">Kealy, T. J.; Pauson, P. L. (1951). "A New Type of Organo-Iron Compound". Nature. 168 (4285): 1039–1040. Bibcode:1951Natur.168.1039K. doi:10.1038/1681039b0. S2CID 4181383. <a href="/wiki/Peter_Pauson" target="_blank">/wiki/Peter_Pauson</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></li>
<li id="fn:12">The 18-valence electron cation [Rh(C5H5)2]+ is called the rhodocenium cation in some journal articles[1] and the rhodicinium cation in others.[11] The former spelling appears more common in more recent literature and so is adopted in this article, but both formulations refer to the same chemical species. <a href="#fnref:12" class="footnote-back-ref">↩</a></li>
<li id="fn:13">Cotton, F. A.; Whipple, R. O.; Wilkinson, G. (1953). "Bis-Cyclopentadienyl Compounds of Rhodium(III) and Iridium(III)". Journal of the American Chemical Society. 75 (14): 3586–3587. doi:10.1021/ja01110a504. <a href="/wiki/F._Albert_Cotton" target="_blank">/wiki/F._Albert_Cotton</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></li>
<li id="fn:14">Mingos, D. M. P. (2001). "A Historical Perspective on Dewar's Landmark Contribution to Organometallic Chemistry". Journal of Organometallic Chemistry. 635 (1–2): 1–8. doi:10.1016/S0022-328X(01)01155-X. <a href="/wiki/Michael_Mingos" target="_blank">/wiki/Michael_Mingos</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></li>
<li id="fn:15">Mehrotra, R. C.; Singh, A. (2007). Organometallic Chemistry: A Unified Approach (2nd ed.). New Delhi: New Age International. pp. 261–267. ISBN 978-81-224-1258-1. Archived from the original on 7 December 2016. Retrieved 15 July 2016. <a href="978-81-224-1258-1" target="_blank">978-81-224-1258-1</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></li>
<li id="fn:16">"The Nobel Prize in Chemistry 1973". Nobel Foundation. Archived from the original on 25 October 2012. Retrieved 12 September 2010. <a href="http://nobelprize.org/nobel_prizes/chemistry/laureates/1973/" target="_blank">http://nobelprize.org/nobel_prizes/chemistry/laureates/1973/</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></li>
<li id="fn:17">Sherwood, Martin (1 November 1973). "Metal Sandwiches". New Scientist. 60 (870): 335. Archived from the original on 3 November 2021. Retrieved 17 June 2017. <a href="/wiki/Martin_Sherwood" target="_blank">/wiki/Martin_Sherwood</a> <a href="#fnref:17" class="footnote-back-ref">↩</a></li>
<li id="fn:18">Cotton, F. A.; Whipple, R. O.; Wilkinson, G. (1953). "Bis-Cyclopentadienyl Compounds of Rhodium(III) and Iridium(III)". Journal of the American Chemical Society. 75 (14): 3586–3587. doi:10.1021/ja01110a504. <a href="/wiki/F._Albert_Cotton" target="_blank">/wiki/F._Albert_Cotton</a> <a href="#fnref:18" class="footnote-back-ref">↩</a></li>
<li id="fn:19">Jacobson, D. B.; Byrd, G. D.; Freiser, B. S. (1982). "Generation of Titanocene and Rhodocene Cations in the Gas Phase by a Novel Metal-Switching Reaction". Journal of the American Chemical Society. 104 (8): 2320–2321. doi:10.1021/ja00372a041. <a href="/wiki/Journal_of_the_American_Chemical_Society" target="_blank">/wiki/Journal_of_the_American_Chemical_Society</a> <a href="#fnref:19" class="footnote-back-ref">↩</a></li>
<li id="fn:20">He, H. T. (1999). Synthesis and Characterisation of Metallocenes Containing Bulky Cyclopentadienyl Ligands (PhD thesis). University of Sydney. OCLC 222646266.{{cite book}}: CS1 maint: location missing publisher (link) <a href="/wiki/University_of_Sydney" target="_blank">/wiki/University_of_Sydney</a> <a href="#fnref:20" class="footnote-back-ref">↩</a></li>
<li id="fn:21">Collins, J. E.; Castellani, M. P.; Rheingold, A. L.; Miller, E. J.; Geiger, W. E.; Rieger, A. L.; Rieger, P. H. (1995). "Synthesis, Characterization, and Molecular-Structure of Bis(tetraphenylcyclopentadienyl)rhodium(II)". Organometallics. 14 (3): 1232–1238. doi:10.1021/om00003a025. <a href="/wiki/Organometallics" target="_blank">/wiki/Organometallics</a> <a href="#fnref:21" class="footnote-back-ref">↩</a></li>
<li id="fn:22">Connelly, N. G.; Geiger, W. E. (1996). "Chemical Redox Agents for Organometallic Chemistry". Chemical Reviews. 96 (2): 877–910. doi:10.1021/cr940053x. PMID 11848774. <a href="/wiki/Chemical_Reviews" target="_blank">/wiki/Chemical_Reviews</a> <a href="#fnref:22" class="footnote-back-ref">↩</a></li>
<li id="fn:23">Pruchnik, F. P. (2005). "45Rh – Rhodium in Medicine". In Gielen, M.; Tiekink, E. R. T (eds.). Metallotherapeutic Drugs and Metal-Based Diagnostic Agents: The Use of Metals in Medicine. Hoboken, NJ: Wiley. pp. 379–398. doi:10.1002/0470864052.ch20. ISBN 0-470-86403-6. <a href="0-470-86403-6" target="_blank">0-470-86403-6</a> <a href="#fnref:23" class="footnote-back-ref">↩</a></li>
<li id="fn:24">Wenzel, M.; Wu, Y. (1988). "Ferrocen-, Ruthenocen-bzw. Rhodocen-analoga von Haloperidol Synthese und Organverteilung nach Markierung mit 103Ru-bzw. 103mRh" [Ferrocene, ruthenocene and rhodocene analogs in haloperidol synthesis and organ distribution after labeling with 103Ru and 103mRh]. International Journal of Radiation Applications and Instrumentation A (in German). 39 (12): 1237–1241. doi:10.1016/0883-2889(88)90106-2. PMID 2851003. <a href="/w/index.php?title=International_Journal_of_Radiation_Applications_and_Instrumentation_A&action=edit&redlink=1" target="_blank">/w/index.php?title=International_Journal_of_Radiation_Applications_and_Instrumentation_A&action=edit&redlink=1</a> <a href="#fnref:24" class="footnote-back-ref">↩</a></li>
<li id="fn:25">Wenzel, M.; Wu, Y. F. (1987). "Abtrennung von [103mRh]Rhodocen-Derivaten von den Analogen [103Ru]Ruthenocen-Derivaten und deren Organ-Verteilung" [Separation of [103mRh]rhodocene derivatives from the parent [103Ru]ruthenocene derivatives and their organ distribution]. International Journal of Radiation Applications and Instrumentation A (in German). 38 (1): 67–69. doi:10.1016/0883-2889(87)90240-1. PMID 3030970. <a href="/w/index.php?title=International_Journal_of_Radiation_Applications_and_Instrumentation_A&action=edit&redlink=1" target="_blank">/w/index.php?title=International_Journal_of_Radiation_Applications_and_Instrumentation_A&action=edit&redlink=1</a> <a href="#fnref:25" class="footnote-back-ref">↩</a></li>
<li id="fn:26">Barlow, S.; O'Hare, D. (1997). "Metal–Metal Interactions in Linked Metallocenes". Chemical Reviews. 97 (3): 637–670. doi:10.1021/cr960083v. PMID 11848884. <a href="/wiki/Chemical_Reviews" target="_blank">/wiki/Chemical_Reviews</a> <a href="#fnref:26" class="footnote-back-ref">↩</a></li>
<li id="fn:27">Wagner, M. (2006). "A New Dimension in Multinuclear Metallocene Complexes". Angewandte Chemie International Edition. 45 (36): 5916–5918. doi:10.1002/anie.200601787. PMID 16906602. <a href="/wiki/Angewandte_Chemie_International_Edition" target="_blank">/wiki/Angewandte_Chemie_International_Edition</a> <a href="#fnref:27" class="footnote-back-ref">↩</a></li>
<li id="fn:28">Zeise, W. C. (1831). "Von der Wirkung zwischen Platinchlorid und Alkohol, und von den dabei entstehenden neuen Substanzen" [On the interaction between platinum chloride and alcohol, and the new substances thereby formed]. Annalen der Physik (in German). 97 (4): 497–541. Bibcode:1831AnP....97..497Z. doi:10.1002/andp.18310970402. Archived from the original on 6 August 2020. Retrieved 12 September 2020. <a href="/wiki/William_Christopher_Zeise" target="_blank">/wiki/William_Christopher_Zeise</a> <a href="#fnref:28" class="footnote-back-ref">↩</a></li>
<li id="fn:29">Leigh, G. J.; Winterton, N., eds. (2002). "Section D: Transition Metal Complexes of Olefins, Acetylenes, Arenes and Related Isolobal Ligands". Modern Coordination Chemistry: The Legacy of Joseph Chatt. Cambridge, UK: RSC Publishing. pp. 101–110. ISBN 0-85404-469-8. Archived from the original on 7 December 2016. Retrieved 15 July 2016. <a href="0-85404-469-8" target="_blank">0-85404-469-8</a> <a href="#fnref:29" class="footnote-back-ref">↩</a></li>
<li id="fn:30">Hunt, L. B. (1984). "The First Organometallic Compounds: William Christopher Zeise and his Platinum Complexes" (PDF). Platinum Metals Review. 28 (2): 76–83. Archived (PDF) from the original on 24 September 2015. Retrieved 8 January 2011. <a href="http://www.platinummetalsreview.com/pdf/pmr-v28-i2-076-083.pdf" target="_blank">http://www.platinummetalsreview.com/pdf/pmr-v28-i2-076-083.pdf</a> <a href="#fnref:30" class="footnote-back-ref">↩</a></li>
<li id="fn:31">Black, M.; Mais, R. H. B.; Owston, P. G. (1969). "The crystal and molecular structure of Zeise's salt, KPtCl3.C2H4.H2O". Acta Crystallographica B. 25 (9): 1753–1759. doi:10.1107/S0567740869004699. <a href="/wiki/P._G._Owston" target="_blank">/wiki/P._G._Owston</a> <a href="#fnref:31" class="footnote-back-ref">↩</a></li>
<li id="fn:32">Jarvis, J. A. J.; Kilbourn, B. T.; Owston, P. G. (1971). "A Re-determination of the Crystal and Molecular Structure of Zeise's salt, KPtCl3.C2H4.H2O". Acta Crystallographica B. 27 (2): 366–372. doi:10.1107/S0567740871002231. <a href="/wiki/P._G._Owston" target="_blank">/wiki/P._G._Owston</a> <a href="#fnref:32" class="footnote-back-ref">↩</a></li>
<li id="fn:33">Mingos, D. M. P. (2001). "A Historical Perspective on Dewar's Landmark Contribution to Organometallic Chemistry". Journal of Organometallic Chemistry. 635 (1–2): 1–8. doi:10.1016/S0022-328X(01)01155-X. <a href="/wiki/Michael_Mingos" target="_blank">/wiki/Michael_Mingos</a> <a href="#fnref:33" class="footnote-back-ref">↩</a></li>
<li id="fn:34">Winterton, N. (2002). "Some Notes on the Early Development of Models of Bonding in Olefin-Metal Complexes". In Leigh, G. J.; Winterton, N. (eds.). Modern Coordination Chemistry: The Legacy of Joseph Chatt. RSC Publishing. pp. 103–110. ISBN 9780854044696. Archived from the original on 26 January 2020. Retrieved 17 June 2017. <a href="9780854044696" target="_blank">9780854044696</a> <a href="#fnref:34" class="footnote-back-ref">↩</a></li>
<li id="fn:35">Astruc, D. (2007). Organometallic Chemistry and Catalysis. Berlin: Springer. pp. 41–43. ISBN 978-3-540-46128-9. Archived from the original on 15 February 2021. Retrieved 15 July 2016. <a href="978-3-540-46128-9" target="_blank">978-3-540-46128-9</a> <a href="#fnref:35" class="footnote-back-ref">↩</a></li>
<li id="fn:36">Leigh, G. J.; Winterton, N., eds. (2002). "Section D: Transition Metal Complexes of Olefins, Acetylenes, Arenes and Related Isolobal Ligands". Modern Coordination Chemistry: The Legacy of Joseph Chatt. Cambridge, UK: RSC Publishing. pp. 101–110. ISBN 0-85404-469-8. Archived from the original on 7 December 2016. Retrieved 15 July 2016. <a href="0-85404-469-8" target="_blank">0-85404-469-8</a> <a href="#fnref:36" class="footnote-back-ref">↩</a></li>
<li id="fn:37">Astruc, D. (2007). Organometallic Chemistry and Catalysis. Berlin: Springer. pp. 41–43. ISBN 978-3-540-46128-9. Archived from the original on 15 February 2021. Retrieved 15 July 2016. <a href="978-3-540-46128-9" target="_blank">978-3-540-46128-9</a> <a href="#fnref:37" class="footnote-back-ref">↩</a></li>
<li id="fn:38">Kealy, T. J.; Pauson, P. L. (1951). "A New Type of Organo-Iron Compound". Nature. 168 (4285): 1039–1040. Bibcode:1951Natur.168.1039K. doi:10.1038/1681039b0. S2CID 4181383. <a href="/wiki/Peter_Pauson" target="_blank">/wiki/Peter_Pauson</a> <a href="#fnref:38" class="footnote-back-ref">↩</a></li>
<li id="fn:39">Laszlo, P.; Hoffmann, R. (2000). "Ferrocene: Ironclad History or Rashomon Tale?". Angewandte Chemie International Edition. 39 (1): 123–124. doi:10.1002/(SICI)1521-3773(20000103)39:1<123::AID-ANIE123>3.0.CO;2-Z. PMID 10649350. <a href="/wiki/Roald_Hoffman" target="_blank">/wiki/Roald_Hoffman</a> <a href="#fnref:39" class="footnote-back-ref">↩</a></li>
<li id="fn:40">Federman Neto, A.; Pelegrino, A. C.; Darin, V. A. (2004). "Ferrocene: 50 Years of Transition Metal Organometallic Chemistry – From Organic and Inorganic to Supramolecular Chemistry". ChemInform. 35 (43). doi:10.1002/chin.200443242. (Abstract; original published in Trends in Organometallic Chemistry, 4:147–169, 2002) <a href="/wiki/ChemInform" target="_blank">/wiki/ChemInform</a> <a href="#fnref:40" class="footnote-back-ref">↩</a></li>
<li id="fn:41">Wilkinson, G.; Rosenblum, M.; Whiting, M. C.; Woodward, R. B. (1952). "The Structure of Iron Bis-Cyclopentadienyl". Journal of the American Chemical Society. 74 (8): 2125–2126. doi:10.1021/ja01128a527. <a href="/wiki/Geoffrey_Wilkinson" target="_blank">/wiki/Geoffrey_Wilkinson</a> <a href="#fnref:41" class="footnote-back-ref">↩</a></li>
<li id="fn:42">Werner, H. (2008). Landmarks in Organo-Transition Metal Chemistry: A Personal View. New York: Springer Science. pp. 161–163. ISBN 978-0-387-09847-0. Archived from the original on 7 December 2016. Retrieved 15 July 2016. <a href="978-0-387-09847-0" target="_blank">978-0-387-09847-0</a> <a href="#fnref:42" class="footnote-back-ref">↩</a></li>
<li id="fn:43">Fischer, E. O.; Pfab, W. (1952). "Zur Kristallstruktur der Di-Cyclopentadienyl-Verbindungen des zweiwertigen Eisens, Kobalts und Nickels" [On the crystal structure of the dicyclopentadienyl compounds of divalent iron, cobalt and nickel]. Zeitschrift für anorganische und allgemeine Chemie (in German). 7 (6): 377–379. doi:10.1002/zaac.19532740603. <a href="/wiki/Ernst_Otto_Fischer" target="_blank">/wiki/Ernst_Otto_Fischer</a> <a href="#fnref:43" class="footnote-back-ref">↩</a></li>
<li id="fn:44">Eiland, P. F.; Pepinsky, R. (1952). "X-ray Examination of Iron Biscyclopentadienyl". Journal of the American Chemical Society. 74 (19): 4971. doi:10.1021/ja01139a527. <a href="/wiki/Journal_of_the_American_Chemical_Society" target="_blank">/wiki/Journal_of_the_American_Chemical_Society</a> <a href="#fnref:44" class="footnote-back-ref">↩</a></li>
<li id="fn:45">Mehrotra, R. C.; Singh, A. (2007). Organometallic Chemistry: A Unified Approach (2nd ed.). New Delhi: New Age International. pp. 261–267. ISBN 978-81-224-1258-1. Archived from the original on 7 December 2016. Retrieved 15 July 2016. <a href="978-81-224-1258-1" target="_blank">978-81-224-1258-1</a> <a href="#fnref:45" class="footnote-back-ref">↩</a></li>
<li id="fn:46">The 18-valence electron cation [Rh(C5H5)2]+ is called the rhodocenium cation in some journal articles[1] and the rhodicinium cation in others.[11] The former spelling appears more common in more recent literature and so is adopted in this article, but both formulations refer to the same chemical species. <a href="#fnref:46" class="footnote-back-ref">↩</a></li>
<li id="fn:47">Cotton, F. A.; Whipple, R. O.; Wilkinson, G. (1953). "Bis-Cyclopentadienyl Compounds of Rhodium(III) and Iridium(III)". Journal of the American Chemical Society. 75 (14): 3586–3587. doi:10.1021/ja01110a504. <a href="/wiki/F._Albert_Cotton" target="_blank">/wiki/F._Albert_Cotton</a> <a href="#fnref:47" class="footnote-back-ref">↩</a></li>
<li id="fn:48">Cotton, F. A.; Whipple, R. O.; Wilkinson, G. (1953). "Bis-Cyclopentadienyl Compounds of Rhodium(III) and Iridium(III)". Journal of the American Chemical Society. 75 (14): 3586–3587. doi:10.1021/ja01110a504. <a href="/wiki/F._Albert_Cotton" target="_blank">/wiki/F._Albert_Cotton</a> <a href="#fnref:48" class="footnote-back-ref">↩</a></li>
<li id="fn:49">"The Nobel Prize in Chemistry 1973". Nobel Foundation. Archived from the original on 25 October 2012. Retrieved 12 September 2010. <a href="http://nobelprize.org/nobel_prizes/chemistry/laureates/1973/" target="_blank">http://nobelprize.org/nobel_prizes/chemistry/laureates/1973/</a> <a href="#fnref:49" class="footnote-back-ref">↩</a></li>
<li id="fn:50">Sherwood, Martin (1 November 1973). "Metal Sandwiches". New Scientist. 60 (870): 335. Archived from the original on 3 November 2021. Retrieved 17 June 2017. <a href="/wiki/Martin_Sherwood" target="_blank">/wiki/Martin_Sherwood</a> <a href="#fnref:50" class="footnote-back-ref">↩</a></li>
<li id="fn:51">Pavlishchuk, V. V.; Addison, A. W. (2000). "Conversion Constants for Redox Potentials Measured Versus Different Reference Electrodes in Acetonitrile Solutions at 25 °C". Inorganica Chimica Acta. 298 (1): 97–102. doi:10.1016/S0020-1693(99)00407-7. <a href="/wiki/Inorganica_Chimica_Acta" target="_blank">/wiki/Inorganica_Chimica_Acta</a> <a href="#fnref:51" class="footnote-back-ref">↩</a></li>
<li id="fn:52">El Murr, N.; Sheats, J. E.; Geiger, W. E.; Holloway, J. D. L. (1979). "Electrochemical Reduction Pathways of the Rhodocenium Ion. Dimerization and Reduction of Rhodocene". Inorganic Chemistry. 18 (6): 1443–1446. doi:10.1021/ic50196a007. <a href="/wiki/Inorganic_Chemistry_(journal)" target="_blank">/wiki/Inorganic_Chemistry_(journal)</a> <a href="#fnref:52" class="footnote-back-ref">↩</a></li>
<li id="fn:53">El Murr, N.; Sheats, J. E.; Geiger, W. E.; Holloway, J. D. L. (1979). "Electrochemical Reduction Pathways of the Rhodocenium Ion. Dimerization and Reduction of Rhodocene". Inorganic Chemistry. 18 (6): 1443–1446. doi:10.1021/ic50196a007. <a href="/wiki/Inorganic_Chemistry_(journal)" target="_blank">/wiki/Inorganic_Chemistry_(journal)</a> <a href="#fnref:53" class="footnote-back-ref">↩</a></li>
<li id="fn:54">El Murr, N.; Sheats, J. E.; Geiger, W. E.; Holloway, J. D. L. (1979). "Electrochemical Reduction Pathways of the Rhodocenium Ion. Dimerization and Reduction of Rhodocene". Inorganic Chemistry. 18 (6): 1443–1446. doi:10.1021/ic50196a007. <a href="/wiki/Inorganic_Chemistry_(journal)" target="_blank">/wiki/Inorganic_Chemistry_(journal)</a> <a href="#fnref:54" class="footnote-back-ref">↩</a></li>
<li id="fn:55">Cotton, F. A.; Whipple, R. O.; Wilkinson, G. (1953). "Bis-Cyclopentadienyl Compounds of Rhodium(III) and Iridium(III)". Journal of the American Chemical Society. 75 (14): 3586–3587. doi:10.1021/ja01110a504. <a href="/wiki/F._Albert_Cotton" target="_blank">/wiki/F._Albert_Cotton</a> <a href="#fnref:55" class="footnote-back-ref">↩</a></li>
<li id="fn:56">Kotz, J. C.; Treichel, P. M.; Townsend, J. R. (2009). Chemistry and Chemical Reactivity, Volume 2 (7th ed.). Belmont, CA: Cengage Learning. pp. 1050–1053. ISBN 978-0-495-38703-9. <a href="978-0-495-38703-9" target="_blank">978-0-495-38703-9</a> <a href="#fnref:56" class="footnote-back-ref">↩</a></li>
<li id="fn:57">Kotz, J. C.; Treichel, P. M.; Townsend, J. R. (2009). Chemistry and Chemical Reactivity, Volume 2 (7th ed.). Belmont, CA: Cengage Learning. pp. 1050–1053. ISBN 978-0-495-38703-9. <a href="978-0-495-38703-9" target="_blank">978-0-495-38703-9</a> <a href="#fnref:57" class="footnote-back-ref">↩</a></li>
<li id="fn:58">Kealy, T. J.; Pauson, P. L. (1951). "A New Type of Organo-Iron Compound". Nature. 168 (4285): 1039–1040. Bibcode:1951Natur.168.1039K. doi:10.1038/1681039b0. S2CID 4181383. <a href="/wiki/Peter_Pauson" target="_blank">/wiki/Peter_Pauson</a> <a href="#fnref:58" class="footnote-back-ref">↩</a></li>
<li id="fn:59">Fischer, E. O.; Pfab, W. (1952). "Zur Kristallstruktur der Di-Cyclopentadienyl-Verbindungen des zweiwertigen Eisens, Kobalts und Nickels" [On the crystal structure of the dicyclopentadienyl compounds of divalent iron, cobalt and nickel]. Zeitschrift für anorganische und allgemeine Chemie (in German). 7 (6): 377–379. doi:10.1002/zaac.19532740603. <a href="/wiki/Ernst_Otto_Fischer" target="_blank">/wiki/Ernst_Otto_Fischer</a> <a href="#fnref:59" class="footnote-back-ref">↩</a></li>
<li id="fn:60">Cotton, F. A.; Whipple, R. O.; Wilkinson, G. (1953). "Bis-Cyclopentadienyl Compounds of Rhodium(III) and Iridium(III)". Journal of the American Chemical Society. 75 (14): 3586–3587. doi:10.1021/ja01110a504. <a href="/wiki/F._Albert_Cotton" target="_blank">/wiki/F._Albert_Cotton</a> <a href="#fnref:60" class="footnote-back-ref">↩</a></li>
<li id="fn:61">Kotz, J. C.; Treichel, P. M.; Townsend, J. R. (2009). Chemistry and Chemical Reactivity, Volume 2 (7th ed.). Belmont, CA: Cengage Learning. pp. 1050–1053. ISBN 978-0-495-38703-9. <a href="978-0-495-38703-9" target="_blank">978-0-495-38703-9</a> <a href="#fnref:61" class="footnote-back-ref">↩</a></li>
<li id="fn:62">El Murr, N.; Sheats, J. E.; Geiger, W. E.; Holloway, J. D. L. (1979). "Electrochemical Reduction Pathways of the Rhodocenium Ion. Dimerization and Reduction of Rhodocene". Inorganic Chemistry. 18 (6): 1443–1446. doi:10.1021/ic50196a007. <a href="/wiki/Inorganic_Chemistry_(journal)" target="_blank">/wiki/Inorganic_Chemistry_(journal)</a> <a href="#fnref:62" class="footnote-back-ref">↩</a></li>
<li id="fn:63">Fischer, E. O.; Wawersik, H. (1966). "Über Aromatenkomplexe von Metallen. LXXXVIII. Über Monomeres und Dimeres Dicyclopentadienylrhodium und Dicyclopentadienyliridium und Über Ein Neues Verfahren Zur Darstellung Ungeladener Metall-Aromaten-Komplexe" [Aromatic Complexes of Metals. LXXXVIII. On the Monomers and Dimers Dicyclopentadienylrhodium and Dicyclopentadienyliridium and a New Method for the Preparation of Uncharged Metal-Aromatic Complexes]. Journal of Organometallic Chemistry (in German). 5 (6): 559–567. doi:10.1016/S0022-328X(00)85160-8. <a href="/wiki/Ernst_Otto_Fischer" target="_blank">/wiki/Ernst_Otto_Fischer</a> <a href="#fnref:63" class="footnote-back-ref">↩</a></li>
<li id="fn:64">Keller, H. J.; Wawersik, H. (1967). "Spektroskopische Untersuchungen an Komplexverbindungen. VI. EPR-spektren von (C5H5)2Rh und (C5H5)2Ir" [Spectroscopic studies of complex compounds. VI. EPR spectra of (C5H5)2Rh and (C5H5)2Ir]. Journal of Organometallic Chemistry (in German). 8 (1): 185–188. doi:10.1016/S0022-328X(00)84718-X. <a href="/wiki/Journal_of_Organometallic_Chemistry" target="_blank">/wiki/Journal_of_Organometallic_Chemistry</a> <a href="#fnref:64" class="footnote-back-ref">↩</a></li>
<li id="fn:65">El Murr, N.; Sheats, J. E.; Geiger, W. E.; Holloway, J. D. L. (1979). "Electrochemical Reduction Pathways of the Rhodocenium Ion. Dimerization and Reduction of Rhodocene". Inorganic Chemistry. 18 (6): 1443–1446. doi:10.1021/ic50196a007. <a href="/wiki/Inorganic_Chemistry_(journal)" target="_blank">/wiki/Inorganic_Chemistry_(journal)</a> <a href="#fnref:65" class="footnote-back-ref">↩</a></li>
<li id="fn:66">De Bruin, B.; Hetterscheid, D. G. H.; Koekkoek, A. J. J.; Grützmacher, H. (2007). "The Organometallic Chemistry of Rh–, Ir–, Pd–, and Pt–Based Radicals: Higher Valent Species". Progress in Inorganic Chemistry. 55: 247–354. doi:10.1002/9780470144428.ch5. ISBN 978-0-471-68242-4. <a href="978-0-471-68242-4" target="_blank">978-0-471-68242-4</a> <a href="#fnref:66" class="footnote-back-ref">↩</a></li>
<li id="fn:67">Keller, H. J.; Wawersik, H. (1967). "Spektroskopische Untersuchungen an Komplexverbindungen. VI. EPR-spektren von (C5H5)2Rh und (C5H5)2Ir" [Spectroscopic studies of complex compounds. VI. EPR spectra of (C5H5)2Rh and (C5H5)2Ir]. Journal of Organometallic Chemistry (in German). 8 (1): 185–188. doi:10.1016/S0022-328X(00)84718-X. <a href="/wiki/Journal_of_Organometallic_Chemistry" target="_blank">/wiki/Journal_of_Organometallic_Chemistry</a> <a href="#fnref:67" class="footnote-back-ref">↩</a></li>
<li id="fn:68">Fischer, E. O.; Wawersik, H. (1966). "Über Aromatenkomplexe von Metallen. LXXXVIII. Über Monomeres und Dimeres Dicyclopentadienylrhodium und Dicyclopentadienyliridium und Über Ein Neues Verfahren Zur Darstellung Ungeladener Metall-Aromaten-Komplexe" [Aromatic Complexes of Metals. LXXXVIII. On the Monomers and Dimers Dicyclopentadienylrhodium and Dicyclopentadienyliridium and a New Method for the Preparation of Uncharged Metal-Aromatic Complexes]. Journal of Organometallic Chemistry (in German). 5 (6): 559–567. doi:10.1016/S0022-328X(00)85160-8. <a href="/wiki/Ernst_Otto_Fischer" target="_blank">/wiki/Ernst_Otto_Fischer</a> <a href="#fnref:68" class="footnote-back-ref">↩</a></li>
<li id="fn:69">The presence of a mirror plane perpendicular to the C5 ring centroid–metal–ring centroid axis of symmetry suggests an eclipsed rather than a staggered conformation. Free rotation of cyclopentadienyl ligands about this axis is common in metallocenes – in ferrocene, the energy barrier to rotation is ~5 kJ mol−1.[13] Consequently, there would be both staggered and eclipsed rhodocene monomer molecules co-existing, and rapidly interconverting, in the solution. It is only in the solid state that a definitive assignment of staggered or eclipsed conformation is meaningful. <a href="#fnref:69" class="footnote-back-ref">↩</a></li>
<li id="fn:70">De Bruin, B.; Hetterscheid, D. G. H.; Koekkoek, A. J. J.; Grützmacher, H. (2007). "The Organometallic Chemistry of Rh–, Ir–, Pd–, and Pt–Based Radicals: Higher Valent Species". Progress in Inorganic Chemistry. 55: 247–354. doi:10.1002/9780470144428.ch5. ISBN 978-0-471-68242-4. <a href="978-0-471-68242-4" target="_blank">978-0-471-68242-4</a> <a href="#fnref:70" class="footnote-back-ref">↩</a></li>
<li id="fn:71">Zagorevskii, D. V.; Holmes, J. L. (1992). "Observation of Rhodocenium and Substituted-Rhodocenium Ions and their Neutral Counterparts by Mass Spectrometry". Organometallics. 11 (10): 3224–3227. doi:10.1021/om00046a018. <a href="/wiki/Organometallics" target="_blank">/wiki/Organometallics</a> <a href="#fnref:71" class="footnote-back-ref">↩</a></li>
<li id="fn:72">In the rhodocene dimer, the joined cyclopentadiene rings are shown with the H atoms in the "endo" position (i.e. the H's are inside, the other half of the ligands are on the outside). Although this is not based on crystal structure data, it does follow the illustrations provided by El Murr et al.[1] and by Fischer and Wawersik[3] in their discussion of the 1H NMR data they collected. The paper by Collins et al.,[18] shows the H atoms in the "exo" position. <a href="/wiki/Endo-exo_isomerism" target="_blank">/wiki/Endo-exo_isomerism</a> <a href="#fnref:72" class="footnote-back-ref">↩</a></li>
<li id="fn:73">Cotton, S. A. (1997). "Rhodium and Iridium". Chemistry of Precious Metals. London: Blackie Academic and Professional. pp. 78–172. ISBN 0-7514-0413-6. Archived from the original on 29 July 2014. Retrieved 15 July 2016. Both metals exhibit an extensive chemistry, principally in the +3 oxidation state, with +1 also being important, and a significant chemistry of +4 iridium existing. Few compounds are known in the +2 state, in contrast to the situation for cobalt, their lighter homologue (factors responsible include the increased stability of the +3 state consequent upon the greater stabilization of the low spin d6 as 10 Dq increases)." (p. 78) <a href="0-7514-0413-6" target="_blank">0-7514-0413-6</a> <a href="#fnref:73" class="footnote-back-ref">↩</a></li>
<li id="fn:74">Hill, A. F. (2002). Organotransition Metal Chemistry. Cambridge, UK: Royal Society of Chemistry. pp. 4–7. ISBN 0-85404-622-4. <a href="0-85404-622-4" target="_blank">0-85404-622-4</a> <a href="#fnref:74" class="footnote-back-ref">↩</a></li>
<li id="fn:75">Hunt, L. B. (1984). "The First Organometallic Compounds: William Christopher Zeise and his Platinum Complexes" (PDF). Platinum Metals Review. 28 (2): 76–83. Archived (PDF) from the original on 24 September 2015. Retrieved 8 January 2011. <a href="http://www.platinummetalsreview.com/pdf/pmr-v28-i2-076-083.pdf" target="_blank">http://www.platinummetalsreview.com/pdf/pmr-v28-i2-076-083.pdf</a> <a href="#fnref:75" class="footnote-back-ref">↩</a></li>
<li id="fn:76">There are two distinct approaches to electron counting, based on either radical species or ionic species. Using the radical approach, a rhodium centre has 9 electrons irrespective of its oxidation states and a cyclopentadienyl ligand is a 5 electron donor. Using the ionic approach, the cyclopentadienyl ligand is a 6 electron donor and the electron count of the rhodium centre depends on its oxidation state – rhodium(I) is an 8 electron centre, rhodium(II) is a 7 electron centre, and rhodium(III) is a 6 electron centre. The two approaches generally reach the same conclusions but it is important to be consistent in using only one or the other. <a href="/wiki/Electron_counting" target="_blank">/wiki/Electron_counting</a> <a href="#fnref:76" class="footnote-back-ref">↩</a></li>
<li id="fn:77">El Murr, N.; Sheats, J. E.; Geiger, W. E.; Holloway, J. D. L. (1979). "Electrochemical Reduction Pathways of the Rhodocenium Ion. Dimerization and Reduction of Rhodocene". Inorganic Chemistry. 18 (6): 1443–1446. doi:10.1021/ic50196a007. <a href="/wiki/Inorganic_Chemistry_(journal)" target="_blank">/wiki/Inorganic_Chemistry_(journal)</a> <a href="#fnref:77" class="footnote-back-ref">↩</a></li>
<li id="fn:78">Keller, H. J.; Wawersik, H. (1967). "Spektroskopische Untersuchungen an Komplexverbindungen. VI. EPR-spektren von (C5H5)2Rh und (C5H5)2Ir" [Spectroscopic studies of complex compounds. VI. EPR spectra of (C5H5)2Rh and (C5H5)2Ir]. Journal of Organometallic Chemistry (in German). 8 (1): 185–188. doi:10.1016/S0022-328X(00)84718-X. <a href="/wiki/Journal_of_Organometallic_Chemistry" target="_blank">/wiki/Journal_of_Organometallic_Chemistry</a> <a href="#fnref:78" class="footnote-back-ref">↩</a></li>
<li id="fn:79">Cotton, F. A.; Whipple, R. O.; Wilkinson, G. (1953). "Bis-Cyclopentadienyl Compounds of Rhodium(III) and Iridium(III)". Journal of the American Chemical Society. 75 (14): 3586–3587. doi:10.1021/ja01110a504. <a href="/wiki/F._Albert_Cotton" target="_blank">/wiki/F._Albert_Cotton</a> <a href="#fnref:79" class="footnote-back-ref">↩</a></li>
<li id="fn:80">Fischer, E. O.; Wawersik, H. (1966). "Über Aromatenkomplexe von Metallen. LXXXVIII. Über Monomeres und Dimeres Dicyclopentadienylrhodium und Dicyclopentadienyliridium und Über Ein Neues Verfahren Zur Darstellung Ungeladener Metall-Aromaten-Komplexe" [Aromatic Complexes of Metals. LXXXVIII. On the Monomers and Dimers Dicyclopentadienylrhodium and Dicyclopentadienyliridium and a New Method for the Preparation of Uncharged Metal-Aromatic Complexes]. Journal of Organometallic Chemistry (in German). 5 (6): 559–567. doi:10.1016/S0022-328X(00)85160-8. <a href="/wiki/Ernst_Otto_Fischer" target="_blank">/wiki/Ernst_Otto_Fischer</a> <a href="#fnref:80" class="footnote-back-ref">↩</a></li>
<li id="fn:81">Fischer, E. O.; Wawersik, H. (1966). "Über Aromatenkomplexe von Metallen. LXXXVIII. Über Monomeres und Dimeres Dicyclopentadienylrhodium und Dicyclopentadienyliridium und Über Ein Neues Verfahren Zur Darstellung Ungeladener Metall-Aromaten-Komplexe" [Aromatic Complexes of Metals. LXXXVIII. On the Monomers and Dimers Dicyclopentadienylrhodium and Dicyclopentadienyliridium and a New Method for the Preparation of Uncharged Metal-Aromatic Complexes]. Journal of Organometallic Chemistry (in German). 5 (6): 559–567. doi:10.1016/S0022-328X(00)85160-8. <a href="/wiki/Ernst_Otto_Fischer" target="_blank">/wiki/Ernst_Otto_Fischer</a> <a href="#fnref:81" class="footnote-back-ref">↩</a></li>
<li id="fn:82">Green, M. L. H.; Pratt, L.; Wilkinson, G. (1959). "760. A New Type of Transition Metal–Cyclopentadiene Compound". Journal of the Chemical Society: 3753–3767. doi:10.1039/JR9590003753. <a href="/wiki/Geoffrey_Wilkinson" target="_blank">/wiki/Geoffrey_Wilkinson</a> <a href="#fnref:82" class="footnote-back-ref">↩</a></li>
<li id="fn:83">Keller, H. J.; Wawersik, H. (1967). "Spektroskopische Untersuchungen an Komplexverbindungen. VI. EPR-spektren von (C5H5)2Rh und (C5H5)2Ir" [Spectroscopic studies of complex compounds. VI. EPR spectra of (C5H5)2Rh and (C5H5)2Ir]. Journal of Organometallic Chemistry (in German). 8 (1): 185–188. doi:10.1016/S0022-328X(00)84718-X. <a href="/wiki/Journal_of_Organometallic_Chemistry" target="_blank">/wiki/Journal_of_Organometallic_Chemistry</a> <a href="#fnref:83" class="footnote-back-ref">↩</a></li>
<li id="fn:84">Fischer, E. O.; Wawersik, H. (1966). "Über Aromatenkomplexe von Metallen. LXXXVIII. Über Monomeres und Dimeres Dicyclopentadienylrhodium und Dicyclopentadienyliridium und Über Ein Neues Verfahren Zur Darstellung Ungeladener Metall-Aromaten-Komplexe" [Aromatic Complexes of Metals. LXXXVIII. On the Monomers and Dimers Dicyclopentadienylrhodium and Dicyclopentadienyliridium and a New Method for the Preparation of Uncharged Metal-Aromatic Complexes]. Journal of Organometallic Chemistry (in German). 5 (6): 559–567. doi:10.1016/S0022-328X(00)85160-8. <a href="/wiki/Ernst_Otto_Fischer" target="_blank">/wiki/Ernst_Otto_Fischer</a> <a href="#fnref:84" class="footnote-back-ref">↩</a></li>
<li id="fn:85">Keller, H. J.; Wawersik, H. (1967). "Spektroskopische Untersuchungen an Komplexverbindungen. VI. EPR-spektren von (C5H5)2Rh und (C5H5)2Ir" [Spectroscopic studies of complex compounds. VI. EPR spectra of (C5H5)2Rh and (C5H5)2Ir]. Journal of Organometallic Chemistry (in German). 8 (1): 185–188. doi:10.1016/S0022-328X(00)84718-X. <a href="/wiki/Journal_of_Organometallic_Chemistry" target="_blank">/wiki/Journal_of_Organometallic_Chemistry</a> <a href="#fnref:85" class="footnote-back-ref">↩</a></li>
<li id="fn:86">Fischer, E. O.; Wawersik, H. (1966). "Über Aromatenkomplexe von Metallen. LXXXVIII. Über Monomeres und Dimeres Dicyclopentadienylrhodium und Dicyclopentadienyliridium und Über Ein Neues Verfahren Zur Darstellung Ungeladener Metall-Aromaten-Komplexe" [Aromatic Complexes of Metals. LXXXVIII. On the Monomers and Dimers Dicyclopentadienylrhodium and Dicyclopentadienyliridium and a New Method for the Preparation of Uncharged Metal-Aromatic Complexes]. Journal of Organometallic Chemistry (in German). 5 (6): 559–567. doi:10.1016/S0022-328X(00)85160-8. <a href="/wiki/Ernst_Otto_Fischer" target="_blank">/wiki/Ernst_Otto_Fischer</a> <a href="#fnref:86" class="footnote-back-ref">↩</a></li>
<li id="fn:87">Szajek, L. P.; Shapley, J. R. (1991). "Unexpected Synthesis of CpIr(η4-C5H6) and a Proton and Carbon-13 NMR Comparison with its Cobalt and Rhodium Congeners". Organometallics. 10 (7): 2512–2515. doi:10.1021/om00053a066. <a href="/wiki/Organometallics" target="_blank">/wiki/Organometallics</a> <a href="#fnref:87" class="footnote-back-ref">↩</a></li>
<li id="fn:88">Cotton, F. A.; Whipple, R. O.; Wilkinson, G. (1953). "Bis-Cyclopentadienyl Compounds of Rhodium(III) and Iridium(III)". Journal of the American Chemical Society. 75 (14): 3586–3587. doi:10.1021/ja01110a504. <a href="/wiki/F._Albert_Cotton" target="_blank">/wiki/F._Albert_Cotton</a> <a href="#fnref:88" class="footnote-back-ref">↩</a></li>
<li id="fn:89">Kealy, T. J.; Pauson, P. L. (1951). "A New Type of Organo-Iron Compound". Nature. 168 (4285): 1039–1040. Bibcode:1951Natur.168.1039K. doi:10.1038/1681039b0. S2CID 4181383. <a href="/wiki/Peter_Pauson" target="_blank">/wiki/Peter_Pauson</a> <a href="#fnref:89" class="footnote-back-ref">↩</a></li>
<li id="fn:90">Jacobson, D. B.; Byrd, G. D.; Freiser, B. S. (1982). "Generation of Titanocene and Rhodocene Cations in the Gas Phase by a Novel Metal-Switching Reaction". Journal of the American Chemical Society. 104 (8): 2320–2321. doi:10.1021/ja00372a041. <a href="/wiki/Journal_of_the_American_Chemical_Society" target="_blank">/wiki/Journal_of_the_American_Chemical_Society</a> <a href="#fnref:90" class="footnote-back-ref">↩</a></li>
<li id="fn:91">Baghurst, D. R.; Mingos, D. M. P. (1990). "Design and Application of a Reflux Modification for the Synthesis of Organometallic Compounds Using Microwave Dielectric Loss Heating Effects". Journal of Organometallic Chemistry. 384 (3): C57 – C60. doi:10.1016/0022-328X(90)87135-Z. <a href="/wiki/Michael_Mingos" target="_blank">/wiki/Michael_Mingos</a> <a href="#fnref:91" class="footnote-back-ref">↩</a></li>
<li id="fn:92">Baghurst, D. R.; Mingos, D. M. P.; Watson, M. J. (1989). "Application of Microwave Dielectric Loss Heating Effects for the Rapid and Convenient Synthesis of Organometallic Compounds". Journal of Organometallic Chemistry. 368 (3): C43 – C45. doi:10.1016/0022-328X(89)85418-X. <a href="/wiki/Michael_Mingos" target="_blank">/wiki/Michael_Mingos</a> <a href="#fnref:92" class="footnote-back-ref">↩</a></li>
<li id="fn:93">Fischer, E. O.; Wawersik, H. (1966). "Über Aromatenkomplexe von Metallen. LXXXVIII. Über Monomeres und Dimeres Dicyclopentadienylrhodium und Dicyclopentadienyliridium und Über Ein Neues Verfahren Zur Darstellung Ungeladener Metall-Aromaten-Komplexe" [Aromatic Complexes of Metals. LXXXVIII. On the Monomers and Dimers Dicyclopentadienylrhodium and Dicyclopentadienyliridium and a New Method for the Preparation of Uncharged Metal-Aromatic Complexes]. Journal of Organometallic Chemistry (in German). 5 (6): 559–567. doi:10.1016/S0022-328X(00)85160-8. <a href="/wiki/Ernst_Otto_Fischer" target="_blank">/wiki/Ernst_Otto_Fischer</a> <a href="#fnref:93" class="footnote-back-ref">↩</a></li>
<li id="fn:94">De Bruin, B.; Hetterscheid, D. G. H.; Koekkoek, A. J. J.; Grützmacher, H. (2007). "The Organometallic Chemistry of Rh–, Ir–, Pd–, and Pt–Based Radicals: Higher Valent Species". Progress in Inorganic Chemistry. 55: 247–354. doi:10.1002/9780470144428.ch5. ISBN 978-0-471-68242-4. <a href="978-0-471-68242-4" target="_blank">978-0-471-68242-4</a> <a href="#fnref:94" class="footnote-back-ref">↩</a></li>
<li id="fn:95">De Bruin, B.; Hetterscheid, D. G. H.; Koekkoek, A. J. J.; Grützmacher, H. (2007). "The Organometallic Chemistry of Rh–, Ir–, Pd–, and Pt–Based Radicals: Higher Valent Species". Progress in Inorganic Chemistry. 55: 247–354. doi:10.1002/9780470144428.ch5. ISBN 978-0-471-68242-4. <a href="978-0-471-68242-4" target="_blank">978-0-471-68242-4</a> <a href="#fnref:95" class="footnote-back-ref">↩</a></li>
<li id="fn:96">Donovan-Merkert, B. T.; Tjiong, H. I.; Rhinehart, L. M.; Russell, R. A.; Malik, J. (1997). "Facile, Redox-Promoted Formation of Rhodocenium Complexes Bearing the 1,2,3-Tri-tert-butylcyclopentadienyl Ligan". Organometallics. 16 (5): 819–821. doi:10.1021/om9608871. <a href="/wiki/Organometallics" target="_blank">/wiki/Organometallics</a> <a href="#fnref:96" class="footnote-back-ref">↩</a></li>
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<li id="fn:123">Donovan-Merkert, B. T.; Clontz, C. R.; Rhinehart, L. M.; Tjiong, H. I.; Carlin, C. M.; Cundari, Thomas R.; Rheingold, Arnold L.; Guzei, Ilia (1998). "Rhodocenium Complexes Bearing the 1,2,3-Tri-tert-butylcyclopentadienyl Ligand: Redox-Promoted Synthesis and Mechanistic, Structural and Computational Investigations". Organometallics. 17 (9): 1716–1724. doi:10.1021/om9707735. <a href="/wiki/Thomas_R._Cundari" target="_blank">/wiki/Thomas_R._Cundari</a> <a href="#fnref:123" class="footnote-back-ref">↩</a></li>
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<li id="fn:138">There are common abbreviations used for molecular fragments in chemical species: "Me" stands for the methyl group, —CH3; "iPr" stands for the iso-propyl group, —CH(CH3)2; "Ph" stands for the phenyl group, —C6H5; "tBu" stands for the tert-butyl group, —C(CH3)3. <a href="/wiki/Methyl" target="_blank">/wiki/Methyl</a> <a href="#fnref:138" class="footnote-back-ref">↩</a></li>
<li id="fn:139">Gusev, O. V.; Morozovaa, L. N.; Peganovaa, T. A.; Petrovskiia, P. V.; Ustynyuka N. A.; Maitlis, P. M. (1994). "Synthesis of η5-1,2,3,4,5-Pentamethylcyclopentadienyl-Platinum Complexes". Journal of Organometallic Chemistry. 472 (1–2): 359–363. doi:10.1016/0022-328X(94)80223-8. <a href="/wiki/Peter_Maitlis" target="_blank">/wiki/Peter_Maitlis</a> <a href="#fnref:139" class="footnote-back-ref">↩</a></li>
<li id="fn:140">Buchholz, D.; Astruc, D. (1994). "The First Decaisopropylmetallocene – One-Pot Synthesis of [Rh(C5iPr5)2]PF6 from [Rh(C5Me5)2]PF6 by Formation of 20 Carbon–Carbon Bonds". Angewandte Chemie International Edition. 33 (15–16): 1637–1639. doi:10.1002/anie.199416371. <a href="/wiki/Angewandte_Chemie_International_Edition" target="_blank">/wiki/Angewandte_Chemie_International_Edition</a> <a href="#fnref:140" class="footnote-back-ref">↩</a></li>
<li id="fn:141">Buchholz, D.; Astruc, D. (1994). "The First Decaisopropylmetallocene – One-Pot Synthesis of [Rh(C5iPr5)2]PF6 from [Rh(C5Me5)2]PF6 by Formation of 20 Carbon–Carbon Bonds". Angewandte Chemie International Edition. 33 (15–16): 1637–1639. doi:10.1002/anie.199416371. <a href="/wiki/Angewandte_Chemie_International_Edition" target="_blank">/wiki/Angewandte_Chemie_International_Edition</a> <a href="#fnref:141" class="footnote-back-ref">↩</a></li>
<li id="fn:142">Buchholz, D.; Astruc, D. (1994). "The First Decaisopropylmetallocene – One-Pot Synthesis of [Rh(C5iPr5)2]PF6 from [Rh(C5Me5)2]PF6 by Formation of 20 Carbon–Carbon Bonds". Angewandte Chemie International Edition. 33 (15–16): 1637–1639. doi:10.1002/anie.199416371. <a href="/wiki/Angewandte_Chemie_International_Edition" target="_blank">/wiki/Angewandte_Chemie_International_Edition</a> <a href="#fnref:142" class="footnote-back-ref">↩</a></li>
<li id="fn:143">Gusev, O. V.; Morozovaa, L. N.; Peganovaa, T. A.; Petrovskiia, P. V.; Ustynyuka N. A.; Maitlis, P. M. (1994). "Synthesis of η5-1,2,3,4,5-Pentamethylcyclopentadienyl-Platinum Complexes". Journal of Organometallic Chemistry. 472 (1–2): 359–363. doi:10.1016/0022-328X(94)80223-8. <a href="/wiki/Peter_Maitlis" target="_blank">/wiki/Peter_Maitlis</a> <a href="#fnref:143" class="footnote-back-ref">↩</a></li>
<li id="fn:144">He, H. T. (1999). Synthesis and Characterisation of Metallocenes Containing Bulky Cyclopentadienyl Ligands (PhD thesis). University of Sydney. OCLC 222646266.{{cite book}}: CS1 maint: location missing publisher (link) <a href="/wiki/University_of_Sydney" target="_blank">/wiki/University_of_Sydney</a> <a href="#fnref:144" class="footnote-back-ref">↩</a></li>
<li id="fn:145">Collins, J. E.; Castellani, M. P.; Rheingold, A. L.; Miller, E. J.; Geiger, W. E.; Rieger, A. L.; Rieger, P. H. (1995). "Synthesis, Characterization, and Molecular-Structure of Bis(tetraphenylcyclopentadienyl)rhodium(II)". Organometallics. 14 (3): 1232–1238. doi:10.1021/om00003a025. <a href="/wiki/Organometallics" target="_blank">/wiki/Organometallics</a> <a href="#fnref:145" class="footnote-back-ref">↩</a></li>
<li id="fn:146">Cotton, S. A. (1997). "Rhodium and Iridium". Chemistry of Precious Metals. London: Blackie Academic and Professional. pp. 78–172. ISBN 0-7514-0413-6. Archived from the original on 29 July 2014. Retrieved 15 July 2016. Both metals exhibit an extensive chemistry, principally in the +3 oxidation state, with +1 also being important, and a significant chemistry of +4 iridium existing. Few compounds are known in the +2 state, in contrast to the situation for cobalt, their lighter homologue (factors responsible include the increased stability of the +3 state consequent upon the greater stabilization of the low spin d6 as 10 Dq increases)." (p. 78) <a href="0-7514-0413-6" target="_blank">0-7514-0413-6</a> <a href="#fnref:146" class="footnote-back-ref">↩</a></li>
<li id="fn:147">Collins, J. E.; Castellani, M. P.; Rheingold, A. L.; Miller, E. J.; Geiger, W. E.; Rieger, A. L.; Rieger, P. H. (1995). "Synthesis, Characterization, and Molecular-Structure of Bis(tetraphenylcyclopentadienyl)rhodium(II)". Organometallics. 14 (3): 1232–1238. doi:10.1021/om00003a025. <a href="/wiki/Organometallics" target="_blank">/wiki/Organometallics</a> <a href="#fnref:147" class="footnote-back-ref">↩</a></li>
<li id="fn:148">Connelly, N. G.; Geiger, W. E. (1996). "Chemical Redox Agents for Organometallic Chemistry". Chemical Reviews. 96 (2): 877–910. doi:10.1021/cr940053x. PMID 11848774. <a href="/wiki/Chemical_Reviews" target="_blank">/wiki/Chemical_Reviews</a> <a href="#fnref:148" class="footnote-back-ref">↩</a></li>
<li id="fn:149">Stojanovic, R. S.; Bond, A. M. (1993). "Examination of Conditions under which the Reduction of the Cobaltocenium Cation can be used as a Standard Voltammetric Reference Process in Organic and Aqueous Solvents". Analytical Chemistry. 65 (1): 56–64. doi:10.1021/ac00049a012. <a href="/wiki/Analytical_Chemistry_(journal)" target="_blank">/wiki/Analytical_Chemistry_(journal)</a> <a href="#fnref:149" class="footnote-back-ref">↩</a></li>
<li id="fn:150">Collins, J. E.; Castellani, M. P.; Rheingold, A. L.; Miller, E. J.; Geiger, W. E.; Rieger, A. L.; Rieger, P. H. (1995). "Synthesis, Characterization, and Molecular-Structure of Bis(tetraphenylcyclopentadienyl)rhodium(II)". Organometallics. 14 (3): 1232–1238. doi:10.1021/om00003a025. <a href="/wiki/Organometallics" target="_blank">/wiki/Organometallics</a> <a href="#fnref:150" class="footnote-back-ref">↩</a></li>
<li id="fn:151">Collins, J. E.; Castellani, M. P.; Rheingold, A. L.; Miller, E. J.; Geiger, W. E.; Rieger, A. L.; Rieger, P. H. (1995). "Synthesis, Characterization, and Molecular-Structure of Bis(tetraphenylcyclopentadienyl)rhodium(II)". Organometallics. 14 (3): 1232–1238. doi:10.1021/om00003a025. <a href="/wiki/Organometallics" target="_blank">/wiki/Organometallics</a> <a href="#fnref:151" class="footnote-back-ref">↩</a></li>
<li id="fn:152">Mehrotra, R. C.; Singh, A. (2007). Organometallic Chemistry: A Unified Approach (2nd ed.). New Delhi: New Age International. pp. 261–267. ISBN 978-81-224-1258-1. Archived from the original on 7 December 2016. Retrieved 15 July 2016. <a href="978-81-224-1258-1" target="_blank">978-81-224-1258-1</a> <a href="#fnref:152" class="footnote-back-ref">↩</a></li>
<li id="fn:153">Collins, J. E.; Castellani, M. P.; Rheingold, A. L.; Miller, E. J.; Geiger, W. E.; Rieger, A. L.; Rieger, P. H. (1995). "Synthesis, Characterization, and Molecular-Structure of Bis(tetraphenylcyclopentadienyl)rhodium(II)". Organometallics. 14 (3): 1232–1238. doi:10.1021/om00003a025. <a href="/wiki/Organometallics" target="_blank">/wiki/Organometallics</a> <a href="#fnref:153" class="footnote-back-ref">↩</a></li>
<li id="fn:154">Donovan-Merkert, B. T.; Clontz, C. R.; Rhinehart, L. M.; Tjiong, H. I.; Carlin, C. M.; Cundari, Thomas R.; Rheingold, Arnold L.; Guzei, Ilia (1998). "Rhodocenium Complexes Bearing the 1,2,3-Tri-tert-butylcyclopentadienyl Ligand: Redox-Promoted Synthesis and Mechanistic, Structural and Computational Investigations". Organometallics. 17 (9): 1716–1724. doi:10.1021/om9707735. <a href="/wiki/Thomas_R._Cundari" target="_blank">/wiki/Thomas_R._Cundari</a> <a href="#fnref:154" class="footnote-back-ref">↩</a></li>
<li id="fn:155">Clarke, M. J.; Sadler, P. J. (1999). Metallopharmaceuticals: Diagnosis and therapy. Berlin: Springer. ISBN 3-540-65308-2. <a href="3-540-65308-2" target="_blank">3-540-65308-2</a> <a href="#fnref:155" class="footnote-back-ref">↩</a></li>
<li id="fn:156">Jones, C. J.; Thornback, J. (2007). Medicinal Applications of Coordination Chemistry. Cambridge, UK: RSC Publishing. ISBN 978-0-85404-596-9. <a href="978-0-85404-596-9" target="_blank">978-0-85404-596-9</a> <a href="#fnref:156" class="footnote-back-ref">↩</a></li>
<li id="fn:157">Pruchnik, F. P. (2005). "45Rh – Rhodium in Medicine". In Gielen, M.; Tiekink, E. R. T (eds.). Metallotherapeutic Drugs and Metal-Based Diagnostic Agents: The Use of Metals in Medicine. Hoboken, NJ: Wiley. pp. 379–398. doi:10.1002/0470864052.ch20. ISBN 0-470-86403-6. <a href="0-470-86403-6" target="_blank">0-470-86403-6</a> <a href="#fnref:157" class="footnote-back-ref">↩</a></li>
<li id="fn:158">Clarke, M. J. (2002). "Ruthenium Metallopharmaceuticals". Coordination Chemistry Reviews. 232 (1–2): 69–93. doi:10.1016/S0010-8545(02)00025-5. <a href="/wiki/Coordination_Chemistry_Reviews" target="_blank">/wiki/Coordination_Chemistry_Reviews</a> <a href="#fnref:158" class="footnote-back-ref">↩</a></li>
<li id="fn:159">Fouda, M. F. R.; Abd-Elzaher, M. M.; Abdelsamaia, R. A.; Labib, A. A. (2007). "On the Medicinal Chemistry of Ferrocene". Applied Organometallic Chemistry. 21 (8): 613–625. doi:10.1002/aoc.1202. <a href="/wiki/Applied_Organometallic_Chemistry" target="_blank">/wiki/Applied_Organometallic_Chemistry</a> <a href="#fnref:159" class="footnote-back-ref">↩</a></li>
<li id="fn:160">Wenzel, M.; Wu, Y. (1988). "Ferrocen-, Ruthenocen-bzw. Rhodocen-analoga von Haloperidol Synthese und Organverteilung nach Markierung mit 103Ru-bzw. 103mRh" [Ferrocene, ruthenocene and rhodocene analogs in haloperidol synthesis and organ distribution after labeling with 103Ru and 103mRh]. International Journal of Radiation Applications and Instrumentation A (in German). 39 (12): 1237–1241. doi:10.1016/0883-2889(88)90106-2. PMID 2851003. <a href="/w/index.php?title=International_Journal_of_Radiation_Applications_and_Instrumentation_A&action=edit&redlink=1" target="_blank">/w/index.php?title=International_Journal_of_Radiation_Applications_and_Instrumentation_A&action=edit&redlink=1</a> <a href="#fnref:160" class="footnote-back-ref">↩</a></li>
<li id="fn:161">Wenzel, M.; Wu, Y. (1988). "Ferrocen-, Ruthenocen-bzw. Rhodocen-analoga von Haloperidol Synthese und Organverteilung nach Markierung mit 103Ru-bzw. 103mRh" [Ferrocene, ruthenocene and rhodocene analogs in haloperidol synthesis and organ distribution after labeling with 103Ru and 103mRh]. International Journal of Radiation Applications and Instrumentation A (in German). 39 (12): 1237–1241. doi:10.1016/0883-2889(88)90106-2. PMID 2851003. <a href="/w/index.php?title=International_Journal_of_Radiation_Applications_and_Instrumentation_A&action=edit&redlink=1" target="_blank">/w/index.php?title=International_Journal_of_Radiation_Applications_and_Instrumentation_A&action=edit&redlink=1</a> <a href="#fnref:161" class="footnote-back-ref">↩</a></li>
<li id="fn:162">Wenzel, M.; Wu, Y. (1988). "Ferrocen-, Ruthenocen-bzw. Rhodocen-analoga von Haloperidol Synthese und Organverteilung nach Markierung mit 103Ru-bzw. 103mRh" [Ferrocene, ruthenocene and rhodocene analogs in haloperidol synthesis and organ distribution after labeling with 103Ru and 103mRh]. International Journal of Radiation Applications and Instrumentation A (in German). 39 (12): 1237–1241. doi:10.1016/0883-2889(88)90106-2. PMID 2851003. <a href="/w/index.php?title=International_Journal_of_Radiation_Applications_and_Instrumentation_A&action=edit&redlink=1" target="_blank">/w/index.php?title=International_Journal_of_Radiation_Applications_and_Instrumentation_A&action=edit&redlink=1</a> <a href="#fnref:162" class="footnote-back-ref">↩</a></li>
<li id="fn:163">Wenzel, M.; Wu, Y. F. (1987). "Abtrennung von [103mRh]Rhodocen-Derivaten von den Analogen [103Ru]Ruthenocen-Derivaten und deren Organ-Verteilung" [Separation of [103mRh]rhodocene derivatives from the parent [103Ru]ruthenocene derivatives and their organ distribution]. International Journal of Radiation Applications and Instrumentation A (in German). 38 (1): 67–69. doi:10.1016/0883-2889(87)90240-1. PMID 3030970. <a href="/w/index.php?title=International_Journal_of_Radiation_Applications_and_Instrumentation_A&action=edit&redlink=1" target="_blank">/w/index.php?title=International_Journal_of_Radiation_Applications_and_Instrumentation_A&action=edit&redlink=1</a> <a href="#fnref:163" class="footnote-back-ref">↩</a></li>
<li id="fn:164">Pruchnik, F. P. (2005). "45Rh – Rhodium in Medicine". In Gielen, M.; Tiekink, E. R. T (eds.). Metallotherapeutic Drugs and Metal-Based Diagnostic Agents: The Use of Metals in Medicine. Hoboken, NJ: Wiley. pp. 379–398. doi:10.1002/0470864052.ch20. ISBN 0-470-86403-6. <a href="0-470-86403-6" target="_blank">0-470-86403-6</a> <a href="#fnref:164" class="footnote-back-ref">↩</a></li>
<li id="fn:165">Barlow, S.; O'Hare, D. (1997). "Metal–Metal Interactions in Linked Metallocenes". Chemical Reviews. 97 (3): 637–670. doi:10.1021/cr960083v. PMID 11848884. <a href="/wiki/Chemical_Reviews" target="_blank">/wiki/Chemical_Reviews</a> <a href="#fnref:165" class="footnote-back-ref">↩</a></li>
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</ol>

Rhodocene open-in-new

Rhodocene