Heteropolyoxometalate

In contrast to isopolyoxometalates, which contain transition metal and oxygen only, the heteropolyoxometalates contain additional main group oxyanions like phosphate and silicate. 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 heteropoly(oxo)metalates 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 heteropoly(oxo)metalates are more thermally robust than isopoly(oxo)metalates. 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, H₄SiW₁₂O₄₀·nH₂O
• Phosphomolybdic acid, H₃Mo₁₂PO₄₀·nH₂O
• Phosphotungstic acid, H₃W₁₂PO₄₀·nH₂O

Isomerism

The Keggin structure type has 5 isomers, which are obtained by (conceptually) rotating one or more of the four M₃O₁₃ units through 60°.