Protonation, Oligomerization, and Condensation Reactions of Vanadate(V), Molybdate(vi), and Tungstate(vi)

This chapter discusses protonation, oligomerization, and condensation reactions of vanadate(V), molybdate(VI), and tungstate(VI). The elements vanadium, molybdenum, and tungsten in their high oxidation states form the stable oxyanions [VO4]3-, [MoO4]2-, and [WO4]2-. In aqueous solution, these ions are easily protonated and show a strong tendency to form polyoxoanions by oxygen bridging, and the release of water molecules. Depending on pH and concentration, a great variety of ions can form in each case, giving rise to complex systems of simultaneous equilibria, which have proved difficult to characterize with certainty. The molybdate(VI) system, in particular, needs further clarification regarding the existence of some polyions. More kinetic and thermodynamic data would also help to improve the understanding of these systems and lead to a general inclusive explanation of the mechanism of polyoxoanion formation. In this respect, new information about some structural preferences of the different metals in mixed polyoxoanions is a useful addition to known facts regarding polyoxometalate structures.