Transition metal hydroxide complexes

<h2 id="hydroxide-as-a-ligand">Hydroxide as a ligand</h2>
Hydroxide is classified as an X ligand in the <a href="/facts/Covalent_bond_classification_method/tb2L7ECf">Covalent bond classification method</a>. In the <a href="/facts/18-electron_rule/KNfG6tpM">usual electron counting method</a>, it is a one-electron ligand when terminal and a three-electron ligand when doubly bridging.
From the electric structure perspective, hydroxide is a strong pi-donor ligand, akin to fluoride. One consequence is that few polyhydroxide complexes are low spin. Another consequence is that electron-precise hydroxide complexes tend to be rather nucleophilic.<a class="footnote-ref" id="fnref:1" href="#fn:1">1</a><a class="footnote-ref" id="fnref:2" href="#fn:2">2</a>

<h2 id="representative-complexes">Representative complexes</h2>
See also: <a href="/facts/Metal_hydroxide/MhgrNW7V">Metal hydroxide</a>
Many hydroxo complexes are prepared by treating metal halides with hydroxide salts. Hydrolysis of basic ligands (amides, alkyls) also produces hydroxide complexes.<a class="footnote-ref" id="fnref:3" href="#fn:3">3</a>
Only a few homoleptic hydroxide complexes are known. These include the d6 species [Pt(OH)6]2−<a class="footnote-ref" id="fnref:4" href="#fn:4">4</a> and the d0 complexes [Ti(OH)6]2− and [Zr2(OH)8(mu−OH)2]2−.<a class="footnote-ref" id="fnref:5" href="#fn:5">5</a>
Many complexes are known where hydroxide shares the coordination sphere with other ligands, i.e., hydroxide is a <a href="/facts/Bridging_ligand/E4ytba8I">bridging ligand</a>. Examples are {[Co(NH3)3]2(mu-OH)3}3+ and its derivative {[Co(NH3)3(H2O)]2(mu-OH)2}4+.<a class="footnote-ref" id="fnref:6" href="#fn:6">6</a>

<h2 id="reactions">Reactions</h2>
Prominent reactions of metal hydroxides are their acid-base behavior. Protonation of metal hydroxides gives <a href="/facts/Aquo_complex/eKudHVQy">aquo complexes</a>: 

LnM−OH + H+ ⇌ LnM−OH+2 where Ln is the ligand complement on the metal M
Thus, aquo ligand is a <a href="/facts/Weak_acid/vJ4rj6Lu">weak acid</a>, of comparable strength to <a href="/facts/Acetic_acid/uyBKA2Pd">acetic acid</a> (pKa of about 4.8).<a class="footnote-ref" id="fnref:7" href="#fn:7">7</a>
In principle but not very commonly, metal hydroxides undergo deprotonation, yielding <a href="/facts/Oxo_complex/CTR37kYS">oxo complexes</a>:

LnM−OH ⇌ LnM=O− +H+
Characteristically, hydroxide ligands are compact and basic. They tend to function as <a href="/facts/Bridging_ligand/E4ytba8I">bridging ligands</a>. One manifestation of this property is the preponderance of di-and polymetallic hydroxide complexes. A practical consequence of this feature is the tendency of metal aquo complexes to form precipitates of meta hydroxides.

<h3>Bioinorganic chemistry</h3>

<a href="/facts/Hemerythrin/lcdQAmHS">Hemerythrins</a>, <a href="/facts/Protein/Jxmagu3E">proteins</a> responsible for <a href="/facts/Oxygen/acyn6o8r">oxygen</a> (O2) transport in some animals have an diiron hydroxide active site. The hydroxide ligand engages the bound O2 through <a href="/facts/Hydrogen_bonding/85V86IIg">hydrogen bonding</a>.
The nucleophilicity of hydroxo ligands is relevant to the role of some M-OH centers in enzymology. For example, in <a href="/facts/Carbonic_anhydrase/je7bk6fk">carbonic anhydrase</a>, a zinc hydroxide binds <a href="/facts/Carbon_dioxide/xGzpppEp">carbon dioxide</a>:<a class="footnote-ref" id="fnref:8" href="#fn:8">8</a>

LnM−OH + CO2 ⇌ LnMO−O−CO2H
The <a href="/facts/Oxygen_evolving_complex/hGsK7zg9">oxygen evolving complex</a> (OEC) consists of a Mn-Ca-O-OH cluster that is responsible for the biosynthesis of O2. It is proposed that the O-O bond forming step involves a hydroxide ligand.
<a href="/facts/Metalloproteinase/ShDT6dbf">Metalloproteinases</a> catalyze the hydrolysis peptide bond. The catalytic center is such enzymes often involves metal hydroxides.

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

<ol>
<li id="fn:1">Nelson, David J.; Nolan, Steven P. (2017). "Hydroxide complexes of the late transition metals: Organometallic chemistry and catalysis" (PDF). Coordination Chemistry Reviews. 353: 278–294. doi:10.1016/j.ccr.2017.10.012. <a href="https://strathprints.strath.ac.uk/62176/1/Nelson_Nolan_CCR2017_Hydroxide_complexes_of_the_late_transition_metals.pdf" target="_blank">https://strathprints.strath.ac.uk/62176/1/Nelson_Nolan_CCR2017_Hydroxide_complexes_of_the_late_transition_metals.pdf</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></li>
<li id="fn:2">Fulton, J. Robin; Holland, Andrew W.; Fox, Daniel J.; Bergman, Robert G. (2002). "Formation, Reactivity, and Properties of Nondative Late Transition Metal−Oxygen and −Nitrogen Bonds". Accounts of Chemical Research. 35 (1): 44–56. doi:10.1021/ar000132x. PMC 1473979. PMID 11790088. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1473979" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1473979</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></li>
<li id="fn:3">Nelson, David J.; Nolan, Steven P. (2017). "Hydroxide complexes of the late transition metals: Organometallic chemistry and catalysis" (PDF). Coordination Chemistry Reviews. 353: 278–294. doi:10.1016/j.ccr.2017.10.012. <a href="https://strathprints.strath.ac.uk/62176/1/Nelson_Nolan_CCR2017_Hydroxide_complexes_of_the_late_transition_metals.pdf" target="_blank">https://strathprints.strath.ac.uk/62176/1/Nelson_Nolan_CCR2017_Hydroxide_complexes_of_the_late_transition_metals.pdf</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></li>
<li id="fn:4">Bandel, G.; Platte, C.; Trömel, M. (1982). "Ammonium hexahydroxoplatinat(IV) und Strukturverfeinerung für Kalium-hexahydroxoplatinat(IV)". Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry. 38 (5): 1544–1546. Bibcode:1982AcCrB..38.1544B. doi:10.1107/S0567740882006311. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></li>
<li id="fn:5">Lin, Hechun; De Oliveira, Peter W.; Huch, Volker; Veith, Michael (2010). "Hydroxometalates from Anion Exchange Reactions of [BF4]− based Ionic Liquids: Formation of [M(OH)6)]2− (M = Ti, Zr) and [Zr(OH)5]−". Chemistry of Materials. 22 (24): 6518–6523. doi:10.1021/cm101490w. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></li>
<li id="fn:6">Wieghardt, K.; Siebert, H. (1985). μ-Carboxylatodi-μ-Hydroxo-Bis[Triamminecobalt(III)] Complexes. Inorganic Syntheses. Vol. 23. pp. 107–116. doi:10.1002/9780470132548.ch21. ISBN 978-0-471-81873-1. <a href="978-0-471-81873-1" target="_blank">978-0-471-81873-1</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></li>
<li id="fn:7">Lincoln, S. F.; Richens, D. T.; Sykes, A. G. (2003). "Metal Aqua Ions". Comprehensive Coordination Chemistry II. Vol. 1. pp. 515–555. doi:10.1016/B0-08-043748-6/01055-0. ISBN 9780080437484. <a href="9780080437484" target="_blank">9780080437484</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></li>
<li id="fn:8">Parkin, Gerard (2004). "Synthetic Analogues Relevant to the Structure and Function of Zinc Enzymes". Chemical Reviews. 104 (2): 699–768. doi:10.1021/cr0206263. PMID 14871139. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></li>
</ol>

Transition metal hydroxide complexes open-in-new

Transition metal hydroxide complexes