| Polyoxometalates (POM) are series of oligomeric aggregative clusters assembled mainly by the early transition metal ions with oxygen anions as linked bridges. They are one set of inorganic molecules in the vast field of coordination compounds, which characterized by the primary packing unit MOn in which a metallic center M surrounded by some oxygen atoms (seldomly by sulfur, fluorine, bromine atoms, and other p-block elements) with coordination number n = 6 (it can also be 4, 5 or 7, as well). Besides these two kinds of atoms, other atom, which are usually labeled as X and always 4-fold or 6-fold-coordinated, lie in the center of the closed shell of MxOy self-assembled by the MOn units. Depending on whether X is present, POMs can be divided into two types, the isopolyanions (IPA) [MnOy]p– and heteropolyanions (HPA) [XzMnOy]q– (z≤n). Because of the special topological and electronic structures, HPAs exhibit versatile properties including acid, redox, bifunctional and even"pseudo-liquid"characteristics, which make them widely applied as acid and oxidation catalysts, as well as new materials in the fields of homo-/heterogeneous catalytic and environmentally-friendly wood...
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