Transition metal hydroxide complexes
Transition metal hydroxide complexes are coordination complexes containing one or more hydroxide (OH−) ligands. The inventory is very large.
Transition metal hydroxide complexes are coordination complexes containing one or more hydroxide (OH−) ligands. The inventory is very large.
Hydroxide as a ligand
[edit]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.[1][2]
Representative complexes
[edit]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(mu−OH)2]2−.[4]
Many complexes are known where hydroxide shares the coordination sphere with other ligands, i.e., hydroxide is a bridging ligand. Examples are {[Co(NH3)3]2(mu-OH)3}3+ and its derivative {[Co(NH3)3(H2O)]2(mu-OH)2}4+.[5]
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Reactions
[edit]Prominent reactions of metal hydroxides are 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
Thus, aquo ligand is a weak acid, 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 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
[edit]
Active site of hemerythrin before and after oxygenation.
Hemerythrins, proteins responsible for oxygen (O2) transport in some animals have an 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 peptide bond. The catalytic center is such enzymes often involves metal hydroxides.
References
[edit]- ^ a b 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.
- ^ 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.
- ^ 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.
- ^ 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.
- ^ 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.
- ^ 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.
- ^ 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.