PolyoxometalatesUpdate based on the original article by Michael T. Pope,Encyclopedia of Inorganic Chemistry© 2005, John Wiley & Sons, Ltd.
Authors: Pope Michael T. , Kortz Ulrich 
Published: December 2012

The class of complexes known aspolyoxometalates(POMs) is predominantly populated by polyoxoanions of high‐valent (d0, d1) transition metals of groups 5 and 6 that typically incorporate some 5‐50 metal centers in structures of high symmetry. In most cases, the metal atoms occupy {MO6} octahedra that share vertices and edges. However, recent reports of POMs that incorporate square‐planar {PdIIO4} and hexagonal‐bipyramidal {UVIO2(η2− O2)2(OH)2} building blocks indicate that further expansion of the field is in progress. The descriptive chemistry of many polyoxomolybdates and polyoxotungstates dates from the last third of the nineteenth century but it was not until the mid‐twentieth century that structural principles and the aqueous equilibria controlling the formation of POMs from monomeric oxoanions began to be recognized and quantified. That additionalheteroatoms(metallic or nonmetallic) can be incorporated into the structures of, especially, polymolybdates and tungstates accounts for the enormous and rapidly developing variety of structures and possible applications of POMs. Four generaltypesof POM structure, based on the mode of incorporation of the heteroatom(s), are described and it is shown how these can lead to hybrid organo‐POM complexes and to functionalization of POMs. Certain POM structures are reducible to mixed‐valence “heteropoly blue” versions, and those incorporating domains of paramagnetic heteroatoms can exhibit single‐molecule magnetic properties. The possibility that MoVIand WVIcan occupy a seven‐coordinate {O = M(O6)} polyhedron in POMs was first observed in [Mo36O112(H2O)8]8−{Mo36}; subsequent work has led to the development of other macrosized POMs including rings {Mo154}, hollow icosahedral “Keplerates” {Mo60W72}, and the “blue hedgehog” {Mo368}. Aqueous and aqueous‐organic solutions of such macro‐POMs spontaneously form hollow spherical vesicular structures (“blackberries”) with diameters typically 90 nm, with inter‐POM separations of 1 nm, depending upon the dielectric constant of the solvent. The various properties of POMs (such as large size, high molecular weight, solubility in polar and nonpolar solvents, electron and proton transfer and storage capacity, high thermal stability) have sparked applications in several fields, most notably in catalysis, both heterogeneous and homogeneous. Other applications include their use as electron‐dense staining and imaging agents, and their antiviral and antitumoral activity. Unconventional POMs are mainly represented by polyoxopalladates(II) and polyperoxodioxouranates(VI), which are also isolated from aqueous media. Unlike the early transition‐metal‐based POMs, the palladate structures are terminated by heteroatom groups and may also contain an internal heteroatom. The class of polyperoxodioxouranates(VI) isolated from more alkaline media frequently have fullerene‐like structures based on hexagonal bipyramidal bipyramids linked by η2‐peroxo groups and hydroxo, oxalato, diphosphato, or diphosphonato bridges.