Published Online: 15 MAR 2006
Copyright © 2006 John Wiley & Sons, Ltd
Encyclopedia of Inorganic Chemistry
How to Cite
Pope, M. T. 2006. Polyoxometalates. Encyclopedia of Inorganic Chemistry. .
- Published Online: 15 MAR 2006
The elements of groups 5 and 6, especially vanadium, molybdenum, and tungsten in their highest oxidation states, form a vast range of polyoxoanions that can incorporate most other positive-valent elements of the periodic table. The first examples of these complexes were discovered in the latter half of the nineteenth century, but it is only within the last few decades that the extraordinary variety and numbers of polyoxoanion structures, properties, and applications have begun to be recognized. The field has historically been divided into isopoly- and heteropolyanions according to whether one or more than one type of positive-valent element is incorporated, respectively. The largest variety of isopolyanions is found with V, Mo, and W, and the results of speciation studies in aqueous solution are summarized. The structures of heteropolyanions are dependent upon the coordination demands of the heteroatom, and the major types based on 4-, 6-, 8-, and 12-coordination are described. In virtually every case, the V, Mo, and W atoms (addenda atoms) occupy MO6 octahedra with one or two (cis) unshared oxygen atoms. Such octahedra share vertices and edges with their neighbors. Structures in which one or more (adjacent) MO6 octahedra have been ‘removed’ can be prepared by the control of solution pH, and the resulting lacunary anions are used as precursors to generate new, larger structures and derivatives. Since the addenda atoms are in their highest oxidation states (d0), many polyoxoanions can reversibly accept variable numbers of electrons to yield mixed-valence heteropoly ‘blues’ (d0,d1) with delocalized hopping electrons and ‘browns’ (d0,d2) with metal–metal bonds. Several strategies for the synthesis of derivatized and functionalized polyoxoanions with organic and organometallic groups are reviewed since this has been a major thrust in the field in recent years. Also, a recent development has been the discovery of extremely large polyoxoanions with 100 + metal atoms that are readily prepared and are stable in aqueous solution. Examples are the mixed-valence polymolybdate wheels (Mo154 and Mo176) and tungstate(VI) assemblies (W184Ln16). Polyoxoanions have long been used in analytical chemistry, and their use as heterogeneous and homogeneous acid and oxidation catalysts continues to be developed. Other applications, especially in structural biology (imaging, crystallographic phasing) and medicine (antiviral and antitumoral agents) are also important.