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Transition metal hydroxide complexes
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Transition metal hydroxide complexes
Transition metal hydroxide complexes are coordination complexes containing one or more hydroxide (OH−) ligands. The inventory is very large.
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.
Many hydroxo complexes are prepared by treating metal halides with hydroxide salts. Hydrolysis of basic ligands (amides, alkyls) also produces hydroxide complexes.
Only a few homoleptic hydroxide complexes are known. These include the d6 species [Pt(OH)6]2− and the d0 complexes [Ti(OH)6]2− and [Zr2(OH)8(mu−OH)2]2−.
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+.
Prominent reactions of metal hydroxides are their acid-base behavior. Protonation of metal hydroxides gives aquo complexes:
Thus, aquo ligand is a weak acid, of comparable strength to acetic acid (pKa of about 4.8).
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Transition metal hydroxide complexes
Transition metal hydroxide complexes are coordination complexes containing one or more hydroxide (OH−) ligands. The inventory is very large.
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.
Many hydroxo complexes are prepared by treating metal halides with hydroxide salts. Hydrolysis of basic ligands (amides, alkyls) also produces hydroxide complexes.
Only a few homoleptic hydroxide complexes are known. These include the d6 species [Pt(OH)6]2− and the d0 complexes [Ti(OH)6]2− and [Zr2(OH)8(mu−OH)2]2−.
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+.
Prominent reactions of metal hydroxides are their acid-base behavior. Protonation of metal hydroxides gives aquo complexes:
Thus, aquo ligand is a weak acid, of comparable strength to acetic acid (pKa of about 4.8).