Polyoxometallate

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 in their high oxidation states. They are usually colorless or orange, 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 V₂O₅, MoO₃, WO₃ dissolve at high pH to give orthometalates, VO3−
4, MoO2−
4, WO2−
4. For Nb₂O₅ and Ta₂O₅, the nature of the dissolved species is less clear. As the pH is lowered, these orthometalates protonate to give oxide–hydroxide compounds such as W(OH)O−
3 and V(OH)O2−
3. These species condense via the process called olation. Condensation proceeds via loss of water and the formation of M–O–M linkages. An abbreviated condensation sequence illustrated with vanadates is:

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