hydroxokomplex

A hydroxokomplex, also known as a hydroxo complex, is a coordination compound in which one or more hydroxide ligands (OH−) are directly bound to a central metal ion. They form when aqua ligands bound to a metal are deprotonated by hydroxide or when a metal ion reacts with hydroxide in aqueous solution. Hydroxy ligands may remain attached as the solution pH changes, giving rise to a family of species with differing stoichiometries and charges.

In solution, hydroxo complexes arise through stepwise hydrolysis of metal aquations. A typical pattern is M(H2O)n^m+ + OH− ⇌ [M(OH)(H2O)n−1]^(m−1)+ + H2O, with further deprotonation producing [M(OH)2(H2O)n−2]+ and beyond. The exact species and charge depend on the metal, its oxidation state, and coordination environment. Hydroxo ligands can be terminal, bound to a single metal center, or bridging (μ−OH), linking two or more metal centers, which affects structure and reactivity.

Hydroxokomplexes are important in many contexts because their formation governs metal speciation, solubility, and catalytic activity. They are common for transition metals such as iron, chromium, manganese, copper, and vanadium in aqueous systems. Their presence influences processes from metal mobility and precipitation of metal hydroxides to catalytic cycles that involve proton-coupled electron transfer or hydrolysis reactions. Spectroscopic and electrochemical techniques, along with hydrolysis constants, are used to characterize and quantify these species.

The term is typically used descriptively, and many specific hydroxo complexes have been characterized for various metals. It intersects with aquo complexes, oxo species, and μ-hydroxo bridges in broader inorganic and bioinorganic chemistry.