The term polyoxometalate (abbreviated POM) is applied to an extremely large group of generally anionic clusters with frameworks built from transition metal oxo anions linked by shared oxide ions. The term is usually applied to clusters of 3 or more transition metal atoms from group 5 and group 6 in their high oxidation states, (d0 and d1 configuration), e.g. (V(V), Nb(V), Ta(V)), (Mo(VI) and W(VI)).
Historically, the first example, the ammonium phosphomolybdate containing the [PMo12O40]3− ion, was discovered in 1826. The structure of the related phosphotungstate anion was determined in 1934, and is generally called the Keggin structure for its discoverer. In the period following this other fundamental structures, e.g the Wells-Dawson ion, were discovered, and their chemistry and applications as catalysts were determined. Whilst this work still continues new areas of interest have emerged, for example:
the discovery of large, highly symmetric polyoxomolybdates such as the wheel-shaped molybdenum blue anions and spherical keplerates.
numerous hybrid organic/inorganic materials that contain POM cores (see as an example )
new potential applications based on unusual magnetic and optical properties of some POM's
potential medicinal applications, in particular anti-tumoral and anti-viral.
Some structural types are found in many different compounds. The first known example of this was the Keggin ion whose structure was found to be common to both molybdates and tungstates with different central hetero atoms. Examples of some fundamental polyoxometalate structures are shown below. The hexamolybdate ion is an iso-polyoxometalate, the other three are hetero-polyoxometalates. The Keggin and Dawson structures have tetrahedrally coordinated hetero-atoms e.g. P or Si, Anderson structure has an octahedral central atom e.g. Al.
Keggin structure, XM12O40n−
Dawson structure, X2M18O62n−
Anderson structure, XM6O24n−
Build up of polyoxometalate structures
The metal atoms that make up the framework (termed "addenda atoms") are typically Mo, W, and V. When more than one element is present the cluster is called a "mixed addenda" cluster.
The ligands coordinated to metal atoms that together form the bridged framework are usually oxide ions, but other elements, such as S and Br have been substituted for some of the oxide ions. (Note that a sulfur substituted POM is often termed a polyoxothiometalates.) Another development is the use of other ligands e.g. nitrosyl and alkoxy to replace oxide ions.
The typical framework building blocks are polyhedral units, with 4, 5, 6 or 7 coordinate metal centres. These units usually share edges and/or vertices, (face sharing is uncommon, but not unknown, for example the ion CeMo12O428− has face shared octahedra with Mo atoms at the vertices of an icosahedron).
The most common unit for polymolybdates is the octahedral MoO6 unit, which is a distorted octahedron where the Mo atom moves off centre to give one short Mo-O bond. In some polymolybdates there are pentagonal bipyramidal units, and these are key building blocks in the molybdenum blues.
Hetero atoms are present in many polyoxometalates. Many different elements can act as hetero-atoms. Examples of various coordination numbers around the hetero-atom are known:
4 coordinate (tetrahedral) in Keggin, Dawson and hexamolybdate structures (e.g. PO4, SiO4, AsO4)
6 coordinate (octahedral) in Anderson structure (e.g. Al(OH)6, TeO6
8 coordinate (square antiprism) in ((CeO8)W10O28)8−
12 coordinate (icosahedral) in (UO12)Mo12O308−
Often the hetero-atom is centrally located in the anion (e.g. Keggin structure) or in a structure fragment e.g. the 2 phosphorus atoms in the Dawson ion are central to the two symmetric fragments.
There are similarities to clathrate structures. The Keggin ion can be formulated as PO42−@ M12O36 and the Dawson as (XO42-)2@M18O54.
Structural isomerism is common. For example the Keggin structure has 5 isomers which can be considered to contain one or more of the four M3O13 units being rotated through 60°.
Many compounds share the same framework architectures or frameworks derived from a larger framework with one or more addenda atoms and attendant oxide ions removed, to give defect structure, usually called a "lacunary" structure. An example of a compound with a Dawson lacunary structure is As2W15O56.
Some cage structures containing ions are known, e.g.. An example is the vanadate cage, V18O 42 containing a Cl−.ion . This has 5 coordinate, square pyramidal vanadium units linked together. H4V18O42 cage containing Cl
Polyoxometalates outside Group 5 and 6
Polyoxoalkoxometalates of titanium and iron are known, e.g. Til2Ol6(OPri)16,the dodecatitanates  and iron oxoalkoxometalate . It is a moot point as to whether these should be categorised as POM's .
Properties and Applications
The huge range of size, structure and elemental composition of known polyoxometalates leads to a wide range of different properties.The Keggin ions are well known to be thermally stable, to be reversibly reduced (by accepting electrons) and are used as catalysts for a range of organic reactions. Some potential "green" applications have been reported e.g a non-chlorine based, wood pulp bleaching process and a method of decontaminating water. Some structures containing transition metal atoms with unpaired electrons have unusual magnetic properties and are being investigated as nano computer storage devices (see qubits).. Some compounds exhibit luminescence . There many reported potential medicinal applications e.g. anti tumoral and anti-viral 
^ abFrom Scheele and Berzelius to Müller: polyoxometalates (POMs) revisited and the "missing link" between the bottom up and top down approaches P. Gouzerh, M. Che; L’Actualité Chimique, 2006, 298, 9
^The Structure and Formula of 12-Phosphotungstic Acid J.F. Keggin. Proc. Roy. Soc., A, 144, 851, 75-100 (1934) doi:10.1098/rspa.1934.0035
^A novel 3D organic–inorganic hybrid based on sandwich-type cadmium hetereopolymolybdate: [Cd4(H2O)2(2,2′-bpy)2] Cd[Mo6O12(OH)3(PO4)2(HPO4)2]2 [Mo2O4(2,2′-bpy)2]2·3H2O Hong-Xu Guo and Shi-Xiong Liu Inorganic Chemistry Communications 7, 11, (2004), 1217 doi:10.1016/j.inoche.2004.09.010
^Classical and Quantum Magnetism in Giant Keplerate Magnetic Molecules Achim Müller, Marshall Luban, Robert Modler, Paul Kögerler, Maria Axenovich, Jürgen Schnack, Paul Canfield, Sergey Bud'ko, Neil Harrison. ChemPhysChem 2001, 2, 517.
^Field-dependent magnetic parameters in Ni4Mo12: Magnetostriction at the molecular level? Jürgen Schnack, Mirko Brüger, Marshall Luban, Paul Kögerler, Emilia Morosan, Ronald Fuchs, Robert Modler, Hiroyuki Nojiri, Ram C. Rai, Jinbo Cao, Janice L. Musfeldt,
Xing Wei. Phys. Rev. B 2006, 73, 094401.
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^ abPolyoxometalates: From Platonic Solids to Anti-Retroviral Activity By Michael Thor Pope, Achim Müller Springer (1994) ISBN 0792324218
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^Polyoxovanadates: High-Nuclearity Spin Clusters with Interesting Host-Guest Systems and Different Electron Populations. Synthesis, Spin Organization, Magnetochemistry, and Spectroscopic Studies A Müller, R Sessoli,E Krickemeyer, H Bögge, J Meyer, D Gatteschi, L Pardi, J Westphal, K Hovemeier, R Rohlfing, J Döring, F Hellweg, C Beugholt and M Schmidtmann Inorg. Chem., 36 (23), 5239 -5250, 1997.
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