Polyoxometalates are polyanionic aggregates comprised of early transition addenda atoms and oxide anions. Addenda atoms are fully oxidized to d 0 electron configurations capable of forming various topologies employing {MOx} units as the principal building block. Conventionally, POM based clusters are formed from the acidification of aqueous molybdate or tungstate oxoanions. The controlled pH treatment of polyoxo species under strictly controlled conditions results in the formation of lacunary anions, which are formed by partial decomposition of their parent compounds. These complexes have formally lost one M=O vertex and possess reactive cavities with high charge density around the defective region due to the negatively charged oxygen ligands. The defective region can react with peripheral metal cations forming a new class of compounds known as transition-metalsubstituted polyoxotungstates, [M(L)XW11O39] q-(X = e.g. P(V), Si(IV), L = H2O, DMSO, etc.).