28.2.7—Ligand exchange reactions

Ligand exchange is a substitution at a metal centre: one ligand leaves as another donates a lone pair. The metal oxidation state may stay the same while colour and geometry change.

Use the ligand and stoichiometry to balance the equation. Copper(II) and cobalt(II) complexes show exchange with water, ammonia, hydroxide or chloride.

Adding excess NH₃ to [Cu(H₂O)₆]²⁺ forms a deep-blue ammine complex; adding OH⁻ can produce Cu(OH)₂ instead. The observations depend on ligand amount and medium.

Ligand exchange is not automatically redox and a colour change does not prove the metal changed oxidation state.