chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...

, the heteropolymetalates are a subset of the polyoxometalates, which consist of three or more transition metal oxyanions linked together by shared oxygen atoms to form a closed 3-dimensional molecular framework. In contrast to isopolymetalates, which contain only one kind of metal atom, the heteropolymetalates contain differing main group oxyanions. The metal atoms are usually group 6 (Mo, W) or less commonly group 5 (V, Nb, Ta) transition metals in their highest oxidation states. They are usually colorless to orange, diamagnetic anions. For most heteropolymetalates the W, Mo, or V, is complemented by main group oxyanions phosphate and

silicate In chemistry, a silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is al ...

. Many exceptions to these general statements exist, and the class of compounds includes hundreds of examples.

Structure

Certain structural motifs recur. The Keggin ion for example is common to both molybdates and tungstates with diverse central heteroatoms. The Keggin and Dawson structures have

tetrahedrally In a tetrahedral molecular geometry, a central atom is located at the center with four substituents that are located at the corners of a tetrahedron. The bond angles are cos−1(−) = 109.4712206...° ≈ 109.5° when all four substituents are ...

-coordinated heteroatoms, such as P or Si, and the Anderson structure has an octahedral central atom, such as aluminium.

Heteropolyacids

Generally, the heteropolymetalates are more thermally robust than homopolymetalates. This trend reflects the stabilizing influence of the tetrahedral oxyanion that "glues" together the transition metal oxo framework. One reflection of their ruggedness, heteropolymetalates can be isolated in their acid form, whereas homopolymetalates typically cannot. Examples include: *

Silicotungstic acid Silicotungstic acid or tungstosilicic acid is a heteropoly acid with the chemical formula . It forms hydrates . In freshly prepared samples, ''n'' is approximately 29, but after prolonged desiccation, ''n'' = 6. It is a white solid although impur ...

, * Phosphomolybdic acid, * Phosphotungstic acid,

Isomerism

The Keggin structure has 5 isomers, which are obtained by (conceptually) rotating one or more of the four units through 60°.

Lacunary structures

The structure of some POMs are derived from a larger POM's structure by removing one or more addenda atoms and their attendant oxide ions, giving a defect structure called a ''lacunary structure''. An example of a compound with a Dawson lacunary structure is . In 2014, vanadate species with similar, selective metal-binding properties were reported.

Uses

This type of acid is a common re-usable acid catalyst in chemical reactions. Dawson ion

The heteropolyacids are widely used as

homogeneous Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the uniformity of a substance or organism. A material or image that is homogeneous is uniform in composition or character (i.e. color, shape, siz ...

and

heterogeneous Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the uniformity of a substance or organism. A material or image that is homogeneous is uniform in composition or character (i.e. color, shape, siz ...

catalysts, particularly those based on the Keggin structure as they can possess qualities such as good thermal stability, high acidity and high oxidising ability. Some examples of catalysis are:"Oxide catalysts in solid state chemistry". T Okuhara, M Misono. ''Encyclopedia of Inorganic Chemistry''. Editor R Bruce King (1994). John Wiley and Sons. *Homogeneous acid catalysis **hydrolysis of propene to give propan-2-ol by and **

Prins reaction The Prins reaction is an organic reaction consisting of an electrophilic addition of an aldehyde or ketone to an alkene or alkyne followed by capture of a nucleophile or elimination of an H+ ion. The outcome of the reaction depends on reaction c ...

by **polymerisation of THF by *Heterogeneous acid catalysis **dehydration of propan-2-ol to propene and

methanol Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is a ...

to hydrocarbons by **reformation of hexane to 2-methylpentane (isohexane) by on *Homogeneous oxidation ** cyclohexene + to adipic acid by the mixed addenda **ketone by to acid and aldehyde by mixed addenda Heteropolyacids have long been used in analysis and histology and are a component of many reagents e.g. the Folin-Ciocalteu reagent, folins phenol reagent used in the Lowry protein assay and EPTA, ethanolic phosphotungstic acid.

Citations

References

* {{Cotton&Wilkinson6th Acid catalysts