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9476 / 2027
Transition Elements

Lesson 5 of 6

Why complexes are coloured

Connect d-orbital orientation to visible-light absorption.

A-Level 9476 (2026-2027)

  • I can explain octahedral d-orbital splitting.Syllabus

    Syllabus 9476, 13(k). Explain octahedral d-orbital splitting.

  • I can explain colour through d-d transitions.Syllabus

    Syllabus 9476, 13(l). Explain colour through d-d transitions.

Octahedral ligands split the five d orbitals

Visible-light absorption can promote an electron across the energy gap.

Make a guess. You are not marked.

Zinc ions in water, [Zn(H2O)6]2+, have the configuration 3d10. What colour is the solution?

  1. Colourless, as its 3d subshell is full
  2. Green, like aqueous iron(II) ions are
  3. Blue, like aqueous copper(II) ions are
Show the answer

Colourless, as its 3d subshell is full

With a full 3d subshell, no electron can be promoted from a lower d orbital into a higher one, so no visible light is absorbed.

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.

Orientation determines the amount of repulsion
OrbitalsShape and orientationOctahedral result
dx2-y2 and dz2The 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 dyzFour 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.

The diagram shows relative energies within an octahedral complex. Splitting is caused by ligand interactions; light then supplies the energy for an electronic transition.

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.

Check your understanding

Why are Cu2+ compounds often coloured, while Zn2+ compounds are usually white?

  1. Cu2+ is d9, so a d-d electron jump can absorb visible light; Zn2+ is d10, with no d-d jump.
  2. Zn2+ is not a metal ion, so it has no d electrons that can absorb any light.
  3. Cu2+ has an empty d subshell, so light passes straight through and makes it look blue.
Show the answer

Cu2+ is d9, so a d-d electron jump can absorb visible light; Zn2+ is d10, with no d-d jump.

Ligands split the d orbitals. In Cu2+ an electron can move to a higher d orbital by absorbing visible light. Zn2+ has a full d subshell, so no d-d transition is possible.