Transition metal hydroxide complexes
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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 perspective of electronic structure, hydroxide is a strong pi-donor ligand, akin to fluoride. One consequence is that few polyhydroxide complexes are low-spin. Another consequence is that in its electron-precise complexes, the hydroxide ligand is nucleophilic.[1][2]
Representative complexes
Many hydroxo complexes are prepared by treating metal halides with hydroxide salts. Hydrolysis of basic ligands (amides, alkyls) also produces hydroxide complexes.[1]
Only a few homoleptic hydroxide complexes are known. These include the d6 species [Pt(OH)6]2−[3] and the d0 complexes [Ti(OH)6]2− and [Zr2(OH)8(μ-OH)2]2−.[4]
Many complexes are known where hydroxide shares the coordination sphere with other ligands, that is, hydroxide is a bridging ligand. Examples are {[Co(NH3)3]2(μ-OH)3}3+ and its derivative {[Co(NH3)3(H2O)]2(μ-OH)2}4+.[5]
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Reactions
A prominent reaction of metal hydroxides involves their acid-base behavior. Protonation of metal hydroxides gives aquo complexes:
- LnM−OH + H+ ⇌ LnM−OH+2 where Ln is the ligand complement on the metal M
Aquo ligands are weak acids, of comparable strength to acetic acid (pKa of about 4.8).[6]
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 dimetallic and polymetallic hydroxide complexes. A practical consequence of this feature is the ready conversion of metal aquo complexes to precipitates of metal-hydroxides.
Bioinorganic chemistry
Active site of hemerythrin before and after oxygenation.
Hemerythrins, proteins responsible for oxygen (O2) transport in some animals have a diiron hydroxide active site. The hydroxide ligand engages the bound O2 through hydrogen bonding.
The nucleophilicity of hydroxo ligands is relevant to the role of some M–OH centers in enzymology. For example, in carbonic anhydrase, a zinc hydroxide binds carbon dioxide:[7]
- LnM−OH + CO2 ⇌ LnMO−O−CO2H
The oxygen evolving complex (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.
Metalloproteinases catalyze the hydrolysis of peptide bonds. The catalytic centers in such enzymes often feature metal hydroxides.