Cobaltocene

<h2 id="synthesis">Synthesis</h2>

<p>Cobaltocene is prepared by the reaction of <a href="/facts/Sodium_cyclopentadienide/l7JaX1NP">sodium cyclopentadienide</a> (NaC5H5) with anhydrous <a href="/facts/Cobalt(II)_chloride/qAAn3wNU">cobalt(II) chloride</a> in <a href="/facts/THF/P5t6SkMK">THF</a> solution.  Sodium chloride is cogenerated, and the <a href="/facts/Organometallic/bhqFz2P0">organometallic</a> product is usually purified by <a href="/facts/Vacuum_sublimation/KDoKokMn">vacuum sublimation</a>.<a class="footnote-ref" id="fnref:1" href="#fn:1"><sup>1</sup></a>
</p>
<h2 id="structure-and-bonding">Structure and bonding</h2>
<p>In Co(C5H5)2 the Co centre is "sandwiched" between two <a href="/facts/Cyclopentadiene/S66eVYvh">cyclopentadienyl</a> (Cp) rings. The Co–C bond lengths are about 2.1 Å, slightly longer than the Fe–C bond in ferrocene.<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a>
</p><p>Co(C5H5)2 belongs to a group of organometallic compounds called <a href="/facts/Metallocene/AecA5Qrk">metallocenes</a> or sandwich compounds.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> Cobaltocene has 19 valence electrons, one more than usually found in organotransition metal complexes such as its very stable relative ferrocene.  (See <a href="/facts/18-electron_rule/KNfG6tpM">18-electron rule</a>.)  This additional electron occupies an orbital that is antibonding with respect to the Co–C bonds.  Consequently, the Co–C distances are slightly longer than the Fe–C bonds in ferrocene.  Many chemical reactions of Co(C5H5)2 are characterized by its tendency to lose this "extra" electron, yielding an 18-electron cation known as cobaltocenium:
</p>

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    {\displaystyle {\ce {\underbrace {2Co(C5H5)2} _{19e-}+I2->\underbrace {2Co(C5H5)2+} _{18e-}+2I-}}}

<p>The otherwise close relative of cobaltocene, <a href="/facts/Rhodocene/XRDdGa8R">rhodocene</a> does not exist as a monomer, but spontaneously dimerizes by formation of a C–C bond between Cp rings.
</p>
<h2 id="reactions">Reactions</h2>
<h3>Redox properties</h3>
<p>Co(C5H5)2 is a common one-electron reducing agent in the laboratory.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a>  In fact, the reversibility of the Co(C5H5)2 <a href="/facts/Oxidation/Pyd5RVcj">redox</a> couple is so well-behaved that Co(C5H5)2 may be used in <a href="/facts/Cyclic_voltammetry/fpTNzaEB">cyclic voltammetry</a> as an <a href="/facts/Internal_standard/YdqEc1vq">internal standard</a>. Its permethylated analogue <a href="/facts/Decamethylcobaltocene/NkYVnSGG">decamethylcobaltocene</a> (Co(C5Me5)2) is an especially powerful reducing agent, due to inductive donation of electron density from the 10 methyl groups, prompting the cobalt to give up its "extra" electron even more so.  These two compounds are rare examples of reductants that dissolve in non-polar organic solvents.  The <a href="/facts/Reduction_potential/PqpNUgSl">reduction potentials</a> of these compounds follow, using the ferrocene-<a href="/facts/Ferrocenium/jKtu9Cp8">ferrocenium</a> couple as the reference:
</p>
<table><tbody><tr><th><a href="/facts/Half-reaction/4pDKxYa6">Half-reaction</a></th><th><i>E</i>0 (V)</th></tr><tr><td>Fe(C5H5)+2 + e− ⇌ Fe(C5H5)2</td><td>0.00 (by definition)</td></tr><tr><td>Fe(C5Me5)+2 + e− ⇌ Fe(C5Me5)2</td><td>−0.59</td></tr><tr><td>Co(C5H5)+2 + e− ⇌ Co(C5H5)2</td><td>−1.33</td></tr><tr><td>Co(C5Me5)+2 + e− ⇌ Co(C5Me5)2</td><td>−1.94</td></tr></tbody></table>
<p>The data show that the decamethyl compounds are around 600 mV more reducing than the parent metallocenes.  This substituent effect is, however, overshadowed by the influence of the metal: changing from Fe to Co renders the reduction more favorable by over 1.3 volts.
</p>
<h3>Carbonylation</h3>
<p>Treatment of Co(C5H5)2 with <a href="/facts/Carbon_monoxide/CKuLtEMJ">carbon monoxide</a> gives the cobalt(I) derivative <a href="/facts/Cyclopentadienylcobalt_dicarbonyl/433dbfJW">Co(C5H5)(CO)2</a>, concomitant with loss of one Cp ligand. This conversion is conducted near 130 °C with 500 psi of CO.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a><a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a>
</p>
<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Rhodocene/XRDdGa8R">Rhodocene</a></li></ul>

<h2 id="external-links">External links</h2>
<ul><li><a href="https://web.archive.org/web/20051012230942/http://www-cie.iarc.fr/htdocs/monographs/vol52/11-cobaltandcobaltcomp.htm">IARC Monograph "Cobalt and Cobalt Compounds"</a></li>
<li><a href="https://web.archive.org/web/20060219230038/http://www.npi.gov.au/database/substance-info/profiles/26.html">National Pollutant Inventory - Cobalt fact sheet</a></li>
<li><a href="http://webbook.nist.gov/chemistry/">NIST Standard Reference Database</a></li></ul>

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

<ol>
<li id="fn:1"><p>King, R. B. (1965). Organometallic Syntheses. Vol. 1. New York, NY: Academic Press. <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>Antipin, M. Yu.; Boese, R.; Augart, N.; Schmid, G. (1993). "Redetermination of the cobaltocene crystal structure at 100 K and 297 K: Comparison with ferrocene and nickelocene". Struct. Chem. 4 (2): 91–101. doi:10.1007/BF00677370. S2CID 93871667. <a href="/w/index.php?title=Structural_Chemistry_(journal)&action=edit&redlink=1" target="_blank">/w/index.php?title=Structural_Chemistry_(journal)&action=edit&redlink=1</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
<li id="fn:3"><p>Elschenbroich, C.; Salzer, A. (1992). Organometallics: A Concise Introduction (2nd ed.). Weinheim: Wiley-VCH. ISBN 978-3-527-28165-7. <a href="978-3-527-28165-7" target="_blank">978-3-527-28165-7</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li>
<li id="fn:4"><p>Connelly, N. G.; Geiger, W. E. (1996). "Chemical Redox Agents for Organometallic Chemistry". Chem. Rev. 96 (2): 877–910. doi:10.1021/cr940053x. PMID 11848774. <a href="/wiki/Chem._Rev." target="_blank">/wiki/Chem._Rev.</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li>
<li id="fn:5"><p>King, R. B. (1965). Organometallic Syntheses. Vol. 1. New York, NY: Academic Press. <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li>
<li id="fn:6"><p>King, R. B.; Stone, F. G. A. (1967). Cyclopentadienyl Metal Carbonyls and Some Derivatives. Inorganic Syntheses. Vol. 7. pp. 99–115. doi:10.1002/9780470132388.ch31. ISBN 9780470132388. <a href="9780470132388" target="_blank">9780470132388</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li>
</ol>

Cobaltocene open-in-new

Cobaltocene