Mixed valence polyanions

We have already discussed the effect that the nature of the solvent and the presence of some cations, particularly quaternary ammonium cations, have on the reactivity of many systems. Silicates are one such system. It is also the case for polyoxometalates, and in particular polyoxovanadates. Furthermore, the formation of V(1V)/V(V) mixed-valence. polyanions also increases the number of structural varieties. Recent studies by A. MUller in Germany and W.G. Klemperer in the United States, have shown spectacular results in the chemistry of polyoxovanadates. In the following sections, the synthesis and structure of some of these compounds arc discussed. 

Recent years have seen the reports of increasing numbers of very large polyoxometalate anions. These may be divided into two classes (i) composite anions constructed of assemblies of simpler, usually lacunary, polyanions and (ii) mixed-valence polymolybdate anions which comprise common building blocks, e.g., Mo8, Mo6, and Mo2 clusters that have no separate, stable existence. 

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