A-Level Properties and complexes of first-row transition metals: worked solution

Question

When ligands coordinate to a copper(II) ion and cause the d-orbitals to split in energy, the resulting complex is coloured. Explain why this is so.

When ligands split the d-orbitals into a lower and a higher energy set separated by an energy gap dE, a d electron can absorb a photon of visible light whose energy equals dE and be promoted from the lower to the higher set (a d-d transition). The wavelength(s) of light absorbed lie in the visible region, so the light transmitted/reflected is the complementary colour, which is the colour we see. Hence the complex is coloured.