28.3.5—Colours of Cu(II) and Co(II) complexes

When a ligand is replaced at a transition-metal centre, the geometry and ligand field can change. The new ΔE changes which visible wavelength is absorbed, so ligand exchange can produce a different solution colour.

Use the ligand, stoichiometry and medium to identify the complex actually present. Water, ammonia, hydroxide and chloride provide contrasting examples for Cu(II) and Co(II).

Adding excess ammonia to a hydrated Cu(II) solution produces a deep-blue ammine complex; hydroxide can instead precipitate Cu(OH)₂. These are different chemical outcomes, not simply “more blue” of the same species.

A colour change does not by itself prove redox. Ligand exchange can leave the oxidation state at +2 while changing coordination, geometry and absorption.