HydroxyLigand

A hydroxy ligand is an oxygen-donor ligand that coordinates to a metal center through a hydroxyl group. In most coordination chemistry contexts, the term refers to a hydroxo ligand, an anionic OH− bound to a metal, typically formed by deprotonation of a coordinated water molecule or by hydrolysis of a metal salt. Neutral M–OH species can occur under certain conditions, but the hydroxo form is most common in aqueous or basic environments.

Coordination modes of hydroxy ligands include terminal binding, where a single M–OH bond is formed, and bridging binding, where one hydroxo group connects two or more metal centers (mu-1,1 or mu-2, etc.). Bridging hydroxo ligands are common in polynuclear complexes and can influence overall geometry, nuclearity, and electronic properties. The M–O bond involving a hydroxo ligand is typically strong and can participate in hydrogen bonding with surrounding ligands or solvent.

Formation and reactivity: Hydroxo ligands arise mainly from deprotonation of aqua ligands or from hydrolysis processes during synthesis or activation of metal complexes. Their presence is often pH-dependent. Hydroxo ligands can act as bases, accepting protons in proton-coupled electron transfer steps, and they can participate in redox chemistry, sometimes undergoing changes in oxidation state during reactions.

Applications and relevance: Hydroxo ligands are common in transition-metal chemistry, including complexes of iron, manganese, copper, and ruthenium. They feature prominently in catalytic systems for water oxidation, hydrolysis reactions, and various oxidation processes, and they appear in bioinorganic contexts as components of dinuclear metal centers and proton-transfer networks.