Highvalent

Highvalent, often written as high-valent, is a term used in chemistry to describe species that possess unusually high oxidation states relative to what is common for a given element. It is not a strict category with fixed boundaries; rather, it is a qualitative descriptor applied across inorganic, organometallic, and bioinorganic contexts. Highvalent species are typically highly reactive and often serve as powerful oxidants. Their existence and stability depend strongly on the ligand environment and the overall coordination geometry around the metal center.

Common examples include manganese in MnO4−, where manganese is in the +7 oxidation state; chromium in CrO4^2−, with chromium in +6; ferrates FeO4^2−, featuring iron in +6; and metal oxides such as OsO4 and RuO4, where osmium and ruthenium are in unusually high oxidation states (+8). High-valent metal–oxo intermediates, such as metal–oxo groups of Mn, Fe, Ru, and others, are central to many oxidation reactions, including organic oxidations and water splitting.

Stability of high-valent states is often enhanced by ligands that are strong π-acceptors or by chelating environments that reduce electron density on the metal. Solvent effects, counterions, and temperature also influence their lifetimes. In biology, high-valent metal states drive challenging transformations, notably in the oxygen-evolving complex of photosystem II, which cycles through high-valent manganese states to oxidize water.

See also: oxidation state, oxidation-reduction potential, high-valent metal-oxo species, hypervalent compounds.