The complex ion \({{\text{[Ni(}}{{\text{H}}_{\text{2}}}{\text{O}}{{\text{)}}_{\text{6}}}{\text{]}}^{2 + }}\) is green and \({{\text{[Ni(N}}{{\text{H}}_{\text{3}}}{{\text{)}}_{\text{6}}}{\text{]}}^{2 + }}\) is blue. Explain why the \({{\text{[Ni(}}{{\text{H}}_{\text{2}}}{\text{O}}{{\text{)}}_{\text{6}}}{\text{]}}^{2 + }}\) complex ion is coloured and outline why changing the identity of the ligand changes the colour of the ion.

Explanation of colour:

d orbitals splits (into two levels);

due to repulsion between d electrons and non-bonding electrons on ligand / due to interaction with electric field of ligands;

difference in energy between levels corresponds to visible light;

(visible light absorbed as) electrons move from lower to higher energy d orbitals;

colour observed complementary to absorbed;

Why changing ligand changes colour:

more electron-dense ligand greater splitting of d orbitals;

3 NH ligand has greater (crystal field/ligand) splitting energy / NH₃ ligand at higher energy in spectrochemical series / OWTTE;

Accept “changing ligand changes d-orbital splitting”.

Surprisingly this was very poorly answered and few students scored all four marks here. There was often no clear understanding of the mechanism of colour absorption (and hence the observed colour being complementary – many candidates thought that the colour was caused by the subsequent emission of energy by electrons falling back). Some of the better candidates did refer to the fact that the ammonia ligand has a greater crystal field/ligand field splitting, which clearly showed comprehensive understanding of this subject.