Occurrence, Stabilization, and Emerging Functions of the Early-Transition Metal Oxyanions Including Polyoxometalates...

Some early-transition metal ions exist in the form of oxyanions, [MO₄]^n-, in which they exhibit variable oxidation states, ranging from +2 to +7, and ultimately carry a charge of ≥1^- in the overall formula. The structure of those oxyanions varies from the tetrahedral [MO₄]^n- monomeric form to octahedral [MO₆]_oh after condensation to 1-D, 2-D, and 3-D functional materials. At acidic pH, early transition metal oxyanions systematically condense to form molecular polynuclear cluster anions, that is, polyoxometalates (POM), where each discrete molecular unit is bridged through oxo/hydroxo ligands. Complexation of [MO₄]^n- with various metal ions expands its geometry, leading to bulk or nanostructures. The metal oxyanion-based compounds are an interesting class of functional materials, among which V-, Mo-, and W-based compounds are well-explored due to their superior lattice stability and ability to form a range of inorganic materials, making them particularly useful for catalytic, magnetic, energy conversion, storage, and sustainable applications. POMs have also emerged as anionic surfactants to stabilize a variety of metal-chalcogenide nanocores, which show excellent functional behaviors. In this perspective, the occurrence of early-transition metal oxyanions, their pH, and aqueous stability have been summarized, which helps to correlate the structure-activity of these frontier materials developed in the last decades.