coordination chemistry A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of chemical bond, bound molecules or ions, that are in turn known as ''ligands'' or complexing ...

, anation is the "replacement of the ligand water by an anion in a coordination entity." The term is however used more loosely to include displacement of any neutral

ligand In coordination chemistry, a ligand is an ion or molecule ( functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's ele ...

by an anion. The reaction is pervasive in coordination chemistry. The reverse reaction, displacement of an anionic ligand by water, is called

aquation Aquation is the chemical reaction involving "incorporation of one or more integral molecules of water" with or without displacement of other atoms or groups. The term is typically employed to refer to reactions of metal complexes where an anion is ...

Example

An example is the conversion of the

aquo complex In chemistry, metal aquo complexes are coordination compounds containing metal ions with only water as a ligand. These complexes are the predominant species in aqueous solutions of many metal salts, such as metal nitrates, sulfates, and perchlora ...

o(NH₃)₅(H₂O)sup>3+ with

bromide A bromide ion is the negatively charged form (Br−) of the element bromine, a member of the halogens group on the periodic table. Most bromides are colorless. Bromides have many practical roles, being found in anticonvulsants, flame-retardan ...

to give pentamminecobalt(III) bromide complex: : o(NH₃)₅(H₂O)sup>3+ + Br⁻ → o(NH₃)₅Brsup>2+ + H₂O The mechanism of such reactions often invoke ion-pairing of the entering anion in the

second coordination sphere In coordination chemistry, the first coordination sphere refers to the array of molecules and ions (the ligands) directly attached to the central metal atom. The second coordination sphere consists of molecules and ions that attached in various ...

followed by dissociation of the aquo ligand.Ghosh, M. C.; Bhattacharya, P.; Banerjee, P., "Anation Reactions of Cobalt(Iii) Complexes", Coordination Chemistry Reviews 1988, 91, 1-34. {{doi, 10.1016/0010-8545(88)80012-2

References

Substitution reactions Coordination chemistry Reaction mechanisms Water chemistry