28.2.1—Ligand reactions forming complexes

Ligands can replace one another around a metal ion. Water, ammonia, hydroxide, chloride and other ligands donate lone pairs and alter charge, colour and geometry.

Balance the metal, ligands and charge in the equation. Excess ligand or precipitating hydroxide can drive a substitution and may produce a different coordination number.

Cu²⁺(aq) + 4NH₃(aq) ⇌ [Cu(NH₃)₄]²⁺ changes the ligand environment and colour; adding OH⁻ can instead form Cu(OH)₂ precipitate.

A colour change does not by itself prove oxidation-state change; ligand substitution can change colour while Cu remains +2.