Heteropolytungstates result from the acidic polycondensation of [WO4-](2-) anions around a heteroelement X. Despite their apparent great diversity, due to the different stereochemical and electronic properties of the heteroatoms, the formation of most heteropolytungstates can be described by analogous successive reactions. In a first step, 9-tungstoheteropolyanions are generally obtained in basic medium and have an open structure. Two types, A and B, and for each type two isomers, alpha and beta, are known. In the next steps which occur at lower pH, closing of the framework or association of the open species directly or through tungsten atoms lead to miscellaneous heteropolytungstates. The reactions are often controlled by the nature of the cationic counter-ions in the solution, for example Na+ or K+. Moreover, in these last steps, the tungsten atoms can be replaced by transition metal cations giving rise to polynuclear complexes. New mono- or polyvacant species can be obtained by selective hydrolytic reactions of the heteropolyanions formed in acid solution, the nature of these species depending again on the nature of the heteroelement and of the counter-ions.
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