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

Question

Under suitable conditions, [V(H2O)6]3+ reacts with zinc to give a solution of a different colour that contains [V(H2O)6]2+ and Zn2+. Account for why the colour of the solution changes.

Colour in transition-metal complexes arises because the d orbitals are split into two energy levels (by the ligands), and light of a particular wavelength is absorbed to promote a d electron from the lower to the higher set (d-d transition); the complementary colour is transmitted/seen. When [V(H2O)6]3+ (V is +3, d2) is reduced by zinc to [V(H2O)6]2+ (V is +2, d3), the number of d electrons changes and the magnitude of the d-orbital splitting (energy gap) changes. Therefore a different wavelength/frequency of visible light is absorbed for the d-d transition, so the complementary colour transmitted is different and the solution colour changes.