In coordination chemistry, anation is the "replacement of the ligand water by an anion in a coordination entity.
"[1] The term is however used more loosely to include displacement of any neutral ligand by an anion.
[2] The reaction is pervasive in coordination chemistry.
The reverse reaction, displacement of an anionic ligand by water, is called aquation.
An example is the conversion of the aquo complex [Co(NH3)5(H2O)]3+ with bromide to give pentamminecobalt(III) bromide complex: The mechanism of such reactions often invoke ion-pairing of the entering anion in the second coordination sphere followed by dissociation of the aquo ligand.