In chemistry, 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. 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-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, 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. The heteropolyacids are widely used as homogeneous and heterogeneous 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: Homogeneous acid catalysis hydrolysis of propene to give propan-2-ol by and Prins reaction by polymerisation of THF by Heterogeneous acid catalysis dehydration of propan-2-ol to propene and methanol 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