polyoxoanionic

Polyoxoanions are a class of inorganic anions characterized by oxygen-rich, metal-oxide clusters that carry multiple negative charges. They are typically built from early transition metals such as molybdenum, tungsten, vanadium, niobium, or tantalum in high oxidation states, linked by shared oxide ligands to form discrete, cluster anions. In solution they can be isolated as salts with various countercations, including protons, alkali metals, or organic cations.

Two major families are recognized: polyoxometalates and related polyoxoanions that may or may not include heteroatoms. The most studied are heteropolyoxoanions (HPAs), where a heteroatom such as phosphorus or silicon sits within the metal-oxide framework, yielding well-defined structures such as the Keggin [XM12O40]n− and the Dawson [X2M18O62]n− anions. Lacunary derivatives arise by removing one or more metal-oxygen units, producing sites for further functionalization.

Structurally, these frameworks consist of corner- and edge-sharing MO6 octahedra. The precise arrangement determines properties such as redox behavior and catalytic activity. Polyoxoanions often tolerate multiple oxidation states, enabling multi-electron transfer processes and versatile reactivity in solution and on surfaces.

Properties and applications: Polyoxoanions are highly anionic and often display significant acidity in aqueous media, along with notable stability under diverse conditions. They serve as catalysts for oxidation and acid-catalyzed reactions, leveraging redox flexibility for homogeneous and heterogeneous catalysis. Derivatives have also been explored in energy storage, sensing, and materials chemistry, and, in some contexts, in biomedical research, though clinical use remains limited and subject to toxicity considerations.

Synthesis typically involves controlled condensation of metal-oxide units under acidic conditions, guiding the formation of stable, discrete anions with specified counterions.