Aquation is the chemical reaction involving "incorporation of one or more integral molecules of water". The term is typically employed to refer to reactions of metal complexes where an anion is displaced by water. For example, bromopentaamminecobalt(III) undergoes the following aquation reaction to give a metal aquo complex:
- [Co(NH3)5Br]2+ + H2O → [Co(NH3)5(H2O)]3+ + Br−
This aquation reaction is catalyzed both by acid and by base. Acid catalysis involves protonation of the bromide, converting it to a better leaving group. Base hydrolysis proceeds by the Sn1CB mechanism, which begins with deprotonation of an ammonia ligand.