• The d sub-level splits into two sets of orbitals of different energy in a complex ion.

• Complexes of d-block elements are coloured, as light is absorbed when an electron is excited between the d-orbitals.

• The colour absorbed is complementary to the colour observed.

• Explanation of the effect of the identity of the metal ion, the oxidation number of the metal and the identity of the ligand on the colour of transition metal ion complexes.

• Explanation of the effect of different ligands on the splitting of the d-orbitals in transition metal complexes and colour observed using the spectrochemical series.

• The spectrochemical series is given in the data booklet in section 15. A list of polydentate ligands is given in the data booklet in section 16.

• Students are not expected to recall the colour of specific complex ions.

• The relation between the colour observed and absorbed is illustrated by the colour wheel in the data booklet in section 17.

• Students are not expected to know the different splitting patterns and their relation to the coordination number. Only the splitting of the 3d orbitals in an octahedral crystal field is required.