Introduction

Abstract

Polyoxometalates (POMs) are anionic molecular metal oxides of early transition metals, mainly tungsten, molybdenum and vanadium, in their highest oxidation states with the general formula [M_xO_y]^n–. They consist of polyhedral building blocks with the formula [MO_z]^m– (z = 4 - 7) which are connected by one (corner-sharing), two (edge-sharing) or seldom three (face-sharing) bridging μ-O atoms as shown in Fig. 1.1.^[1] The transition metal atoms building the cluster framework are called addenda atoms.