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Concept
Polyoxometalate
Summary
In chemistry, a polyoxometalate (abbreviated POM) is a polyatomic ion, usually an anion, that consists of three or more transition metal oxyanions linked together by shared oxygen atoms to form closed 3-dimensional frameworks. The metal atoms are usually group 6 (Mo, W) or less commonly group 5 (V, Nb, Ta) transition metals and Tc in their high oxidation states. Polyoxometalates are often colorless, orange or red diamagnetic anions. Two broad families are recognized, isopolymetalates, composed of only one kind of metal and oxide, and heteropolymetalates, composed of one metal, oxide, and a main group oxyanion (phosphate, silicate, etc.). Many exceptions to these general statements exist.
The oxides of d0 metals such as , , dissolve at high pH to give orthometalates, , , . For and , the nature of the dissolved species at high pH is less clear, but these oxides also form polyoxometalates.
As the pH is lowered, orthometalates protonate to give oxide–hydroxide compounds such as and . These species condense via the process called olation. The replacement of terminal M=O bonds, which in fact have triple bond character, is compensated by the increase in coordination number. The nonobservation of polyoxochromate cages is rationalized by the small radius of Cr(VI), which may not accommodate octahedral coordination geometry.
Condensation of the species entails loss of water and the formation of linkages. The stoichiometry for hexamolybdate is shown:
6 MoO4^2- + 10 HCl -> [Mo6O19]^2- + 10 Cl- + 5 H2O
An abbreviated condensation sequence illustrated with vanadates is:
4 VO4^3- + 8 H+ -> V4O12^4- + 4 H2O
5 V4O12^4- + 12 H+ -> 2 V10O26(OH)2^4- + 4 H2O
When such acidifications are conducted in the presence of phosphate or silicate, heteropolymetalate result. For example, the phosphotungstate anion consists of a framework of twelve octahedral tungsten oxyanions surrounding a central phosphate group.
Ammonium phosphomolybdate, anion, was reported in 1826. The isostructural phosphotungstate anion was characterized by X-ray crystallography 1934.
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