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Transition metal hydroxide complexes

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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.

Hydroxide as a ligand

Hydroxide is classified as an X ligand in the Covalent bond classification 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: [Pt(OH)6], [Ti(OH)6], and [Zr(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:

LnM−OH + H ⇌ LnM−OH+2

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

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

  1. 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.
  2. 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.
  3. Wieghardt, K.; Siebert, H. (1985). "μ-Carboxylatodi-μ-Hydroxo-Bis[Triamminecobalt(III)] Complexes". Inorganic Syntheses. 23: 107–116. doi:10.1002/9780470132548.ch21.
  4. Lincoln, S. F.; Richens, D. T.; Sykes, A. G. (2003). "Metal Aqua Ions". Comprehensive Coordination Chemistry II. Comprehensive Coordination Chemistry II. Vol. 1. pp. 515–555. doi:10.1016/B0-08-043748-6/01055-0. ISBN 9780080437484.
Coordination complexes
H donors:
B donors:
C donors:
Si donors:
N donors:
P donors:
O donors:
S donors:
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