aquation

Aquation is a chemical process in which a water molecule substitutes a ligand bound to a metal center, forming a metal-aqua complex. It occurs predominantly in aqueous solution and is an important step in the ligand substitution chemistry of many transition metal ions and coordination compounds.

The mechanism of aquation can be dissociative, where a leaving ligand departs to create a vacant coordination site before water binds, or associative, where water attacks the metal center to form a higher-coordinate intermediate that subsequently loses a ligand. The rate and pathway of aquation depend on the metal’s identity and oxidation state, the lability of existing ligands, the overall charge, temperature, pH, and the concentration of competing nucleophiles such as chloride or other anions present in solution.

Aquation often precedes further ligand substitution or hydrolysis, influencing the speciation and reactivity of metal ions in biology, environmental chemistry, and catalysis. For many metal ions, the hexaaqua complex [M(H2O)6]n+ is a common intermediate in solution, with successive water ligands replaced by other ligands as the reaction proceeds. In biological and medicinal contexts, aquation can activate metal-based drugs; for example, platinum-based anticancer agents such as cisplatin undergo aquation in aqueous environments, replacing chloride ligands with water to form more reactive species that bind DNA.

In summary, aquation is the water-driven activation of metal complexes through ligand substitution, with broad implications across chemistry, biology, and environmental science.