New trends in polyoxometalate chemistry, toward large polyanions, toward nitrosyl-substituted polyanions

Abstract

The chemistry of polymolybdates and polytungstates demonstrates various facts which may be put together with some value. (1) The variety of compounds is quite large, even if the oxidation number of the metal is maintained at 6 and then provides an octahedral surrounding and a polyanion surface covered by oxygen. (2) Taking the case of molybdenum, and going from water toward non-aqueous solvents, it is clear that many other structures can be found or stabilized by using that type of solvent; also the occurrence of the tetrahedron becomes more frequent. (3) Molybdenum offers another facet of reactivity, due to its easy reduction. (4) The formal oxygen substitution of an MO bond by a nitrogen or an oxygen containing ligand, such as nitrosyl, phenyldiazenido, alcoolate, oximate, amidoximate, an anion deriving from an organic acid, etc., opens the door to a wide range of reactions; moreover it provides some new structural types unknown for the unsubstituted polyanion. The catalytic effect of polyoxometalates has been often underlined and currently is under active study. There is no doubt that the development of polyoxometalate chemistry will help in the understanding of these complex catalytic reactions, and will maybe provide other efficient catalytic procedures.