Isopolyanions

Isopolyanions are a class of polyoxometalate anions composed of a single type of metal-oxide unit linked by oxide bridges. They are a subset of polyoxometalates that lack a heteroatom core, in contrast to heteropolyanions that incorporate a heteroatom such as phosphorus or silicon within the cluster. In isopolyanions the metal centers are typically early transition metals in high oxidation states (most commonly molybdenum(VI) and tungsten(VI), with vanadium-based examples known as well) arranged in various oxygen-bridged frameworks. The name reflects their construction entirely from one metal type and oxide connectivity.

Formation and structure: Isopolyanions form in aqueous solution by condensation of metal-oxide units with loss of water, under controlled pH and often in the presence of counterions to balance charge. They exhibit defined nuclearities and can assemble into larger discrete clusters, ranging from small to more complex three-dimensional frameworks. Many structures are built from connected MO6 octahedra sharing corners and edges, yielding highly anionic, highly charged species that are stable enough to be isolated or remain in solution.

Properties: Isopolyanions are typically highly soluble in water and exhibit rich redox chemistry, enabling multiple oxidation states and potential catalytic activity. They show strong acidity profiles and can act as robust inorganic ions in solution. Their high negative charge and well-defined structures make them useful as models for metal-oxide surfaces and as precursors to solid-state materials.

Applications and examples: Well-characterized members include tungsten- and molybdenum-based isopolyanions, with metatungstate and paratungstate families among the commonly cited examples. Research spans synthesis, structural characterization, redox studies, catalysis, electrochemistry, and materials science, highlighting the utility of isopolyanions as catalysts, reagents, and precursors in inorganic chemistry.