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Transition metal hydroxide complexes are a large family of coordination complexes containing hydroxide (OH) as a ligand. The inventory is very large.
Representative complexes
Homoleptic complexes
Only a few homoleptic hydroxide complexes are known: [Pt(OH)6]−2, [Ti(OH)6]−2, and [Zr(OH)8(mOH)2]−2.
Reactions
Prominent reactions of metal hydroxides are their acid-base behavior. Protonation of metal hydroxides gives aquo complexes:
- LnM−OH + H ⇌ LnM−OH+2
In principle but not very commonly, metal hydroxides undergo deprotonation, yielding oxo complexes:
- LnM−OH ⇌ LnM=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.
The hydroxo ligand is a nucleophile. This behavior is relevant to their role in enzymology. In carbonic anhydrase, a zinc hydroxide binds carbon dioxide:
- LnM−OH + CO2 ⇌ LnMO−O−CO2H
References
- 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.