Polyoxometalate

In chemistry, a polyoxometalate (abbreviated POM) is a polyatomic ion, usually an anion, that consists of three or more transition metal oxyanions linked together by shared oxygen atoms to form closed 3-dimensional frameworks.

For Nb2O5 and Ta2O5, the nature of the dissolved species at high pH is less clear, but these oxides also form polyoxometalates.

The nonobservation of polyoxochromate cages is rationalized by the small radius of Cr(VI), which may not accommodate octahedral coordination geometry.

The stoichiometry for hexamolybdate is shown:[3] An abbreviated condensation sequence illustrated with vanadates is:[1][4] When such acidifications are conducted in the presence of phosphate or silicate, heteropolymetalate result.

For example, the phosphotungstate anion [PW12O40]3− consists of a framework of twelve octahedral tungsten oxyanions surrounding a central phosphate group.

Polyoxometalates typically exhibit coordinate metal-oxo bonds of different multiplicity and strength.

[10] However, metal-metal bonds are not completely absent in polyoxometalates and they are often present among the highly reduced species.

[12] Corresponding ammonium polyoxotechnetate salt was recently isolated from trifluoromethanesulfonic acid and it has very similar structure.

These octaanions form in strongly basic conditions from alkali melts of the extended metal oxides (M2O5), or in the case of Nb even from mixtures of niobic acid and alkali metal hydroxides in aqueous solution.

The anion is in the form of a tire (the cavity has a diameter of more than 20 Å) and an extremely large inner and outer surface.

[20] Lanthanide-containing polyoxometalates show chemoselectivity[21] and are also able to form inorganic–organic adducts, which can be exploited in chiral recognition.

Other ligands replacing the oxide ions have also been demonstrated, such as nitrosyl and alkoxy groups.

POM-based aerobic oxidations have been promoted as alternatives to chlorine-based wood pulp bleaching processes,[37] a method of decontaminating water,[38] and a method to catalytically produce formic acid from biomass (OxFA process).

[40] Some POMs exhibit unusual magnetic properties,[41] which has prompted visions of many applications.

[46] The Anderson-type polyoxomolybdates and heptamolybdates exhibit activity for suppressing the growth of some tumors.

[47][48] The Wells-Dawson structure can efficiently inhibit amyloid β (Aβ) aggregation in a therapeutic strategy for Alzheimer's disease.