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Transition metal hydroxide complexes are coordination complexes containing one or more hydroxide (OH) ligands. The inventory is very large.

Hydroxide as a ligand

Hydroxide is classified as an X ligand in the Covalent bond classification method. In the usual electron counting method, 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.

Representative complexes

Homoleptic complexes

Only a few homoleptic hydroxide complexes are known. These include the d species [Pt(OH)₆] and the d complexes [Ti(OH)₆] and [Zr₂(OH)₈(mu−OH)₂].

Mixed ligand complexes

Many complexes are known where hydroxide shares the coordination sphere with other ligands. One pair of such complexes are {₂(mu-OH)₃} and its derivative {₂(mu-OH)₂}.

Reactions

Prominent reactions of metal hydroxides are their acid-base behavior. Protonation of metal hydroxides gives aquo complexes:

L_nM−OH + H ⇌ L_nM−OH+2 where L_n is the ligand complement on the metal M

Thus, aquo ligand is a weak acid, of comparable strength to acetic acid (pK_a of about 4.8).

In principle but not very commonly, metal hydroxides undergo deprotonation, yielding oxo complexes:

L_nM−OH ⇌ L_nM=O +H

Characteristically, hydroxide ligands are compact and basic. They tend to function as bridging ligands. 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.

Bioinorganic chemistry

The hydroxo ligand is a nucleophile. This behavior is relevant to their role in enzymology. In carbonic anhydrase, a zinc hydroxide binds carbon dioxide:

L_nM−OH + CO₂ ⇌ L_nMO−O−CO₂H

The oxygen evolution complex (OEC), which consists of a Mn-Ca-O-OH cluster that is responsible for making O2, to supply our atmosphere. It is proposed that the O-O bond forming step involves a hydroxide ligand.

Metalloproteinases catalyze the hydrolysis peptide bond. The catalytic center is such enzymes often involves metal hydroxides.

References

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.
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.
3. 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. doi:10.1107/S0567740882006311.
4. 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)] (M = Ti, Zr) and [Zr(OH)₅]". Chemistry of Materials. 22 (24): 6518–6523. doi:10.1021/cm101490w.
5. Wieghardt, K.; Siebert, H. (1985). "μ-Carboxylatodi-μ-Hydroxo-Bis[Triamminecobalt(III)] Complexes". Inorganic Syntheses. 23: 107–116. doi:10.1002/9780470132548.ch21.
6. 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.
7. 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.

• Ligands