Topic 5 of 6
Why complexes are coloured
Connect d-orbital orientation to visible-light absorption.
A-Level 9476 (2026-2027)
Octahedral ligands split the five d orbitals
Visible-light absorption can promote an electron across the energy gap.
For an isolated metal ion, the five d orbitals have equal energy: they are degenerate. In an octahedral complex, six ligands approach along the positive and negative x, y and z axes. Their electron pairs repel d-electron density unevenly because the orbitals point in different directions.
| Orbitals | Shape and orientation | Octahedral result |
|---|---|---|
| dx2-y2 and dz2 | The first has four lobes along x and y. The second has two z-axis lobes and a ring around its middle. Both put substantial density towards axial ligands. | Greater repulsion; the higher-energy pair. |
| dxy, dxz and dyz | Four lobes between the indicated axes, in the xy, xz or yz plane. | Less direct repulsion; the lower-energy set of three. |
Octahedral splitting and a d-d transition
The two d orbitals directed towards axial ligands are higher in energy than the three oriented between axes. Absorbing a photon with energy equal to the gap can promote a d electron from the lower to the upper set.
If a suitable lower d level is occupied and a higher one can receive an electron, light of energy equal to the gap can cause a d-d transition. Absorbing selected visible wavelengths leaves the complementary mixture to be transmitted or reflected, giving the observed colour. A blue solution transmits blue light; it does not look blue because it absorbs blue most strongly.
Changing the ligand, metal or oxidation state can change the splitting and the wavelength absorbed. This explains why ligand exchange can change colour while oxidation state remains fixed. You do not need a memorised ranking of ligand field strengths for this syllabus.