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ed M-O cluster.1 They are discrete multi transition metal oxides characterized by a formidable structural variety, resulting in different dimensions, shape, charge density, surface reactivity, and rich redox chemistry.[2-7] A first classification of POMs is based on the chemical composition of these species, essentially represented by two types of general formula [8]:
Isopolyanions [MaOb]p- (1)
Heteropolyanions [XcMaOb]q-, with c := a (2)
Where M is the main transition metal constituent of the POM, O is the oxygen atom, and X can be a nonmetal of the p block or a different transition metal. There are no chemical limitations for X and M by definition. The X elements are the so- called primary, central or heteroatoms. In general, any element except noble gases can participate as X in a POM cluster as there are no strict physical requirements for this position.[1] On the other hand, M is the so called secondary, peripheral or addenda atom and only some elements are typically found in such compounds. Despite the simplicity of formulae (1) and (2) above, the composition of a cluster can be highly complex, with various M elements taking part in the structure. POMs can be described as composed of MOn units, where n indicates the coordination number (n=4, 5, 6 or 7) of M. usually the distorted octahedral coordination (n=6) is observed. Most of POMs are based on edge- and corner- shared MO6 octahedra, where M is an early transition metal in its highest oxidation state (V+5, Nb+5, Ta+5, Mo+6).
