Why transition metal complexes are coloured

Splitting of d orbitals (spec 17.7, 17.8)

In an isolated ion the five 3d orbitals have the same energy. When ligands bond to the ion, their lone pairs repel the d orbitals unevenly, splitting them into two groups with an energy gap ΔE.

A d electron can absorb a photon of visible light with energy ΔE = hf and jump to the higher level. Light of that frequency is absorbed; the remaining (complementary) colours are transmitted or reflected, and that is the colour we see. [Cu(H2O)6]2+ absorbs in the red-orange region and looks blue.

Colourless ions (spec 17.9)

No d-d transition is possible, so no visible light is absorbed, when:

• the d sub-shell is empty (3d0): Sc3+, Ti4+
• the d sub-shell is full (3d10): Zn2+, Cu+

Their solutions are colourless.

Colour changes (spec 17.11)

The size of ΔE, and so the colour, depends on:

• oxidation number: [Fe(H2O)6]2+ pale green, [Fe(H2O)6]3+ yellow-brown
• ligand: [Cu(H2O)6]2+ pale blue, [Cu(NH3)4(H2O)2]2+ deep blue
• coordination number (shape): [Co(H2O)6]2+ pink (octahedral), [CoCl4]2- blue (tetrahedral)

Worked example

Explain why aqueous copper(II) sulfate is blue but aqueous zinc sulfate is colourless.

1. Cu2+ is 3d9: water ligands split the d orbitals and an electron absorbs red-orange light to move to the higher level; blue light is transmitted.
2. Zn2+ is 3d10: the d sub-shell is full, so no electron can move between d levels.

Answer: Copper(II) absorbs visible light by d-d transitions; zinc(II) cannot

Check yourself

1. Why are most aqueous transition metal ions coloured?
   • The metal ions emit light when heated
   • Electrons move from 4s to 4p
   • The ions reflect all visible light
   • Ligands split the d orbitals, and electrons absorb visible light to move between d levels
   Show answer

   Ligands split the d orbitals, and electrons absorb visible light to move between d levels. Absorption of particular frequencies leaves the complementary colour.

2. Why is a solution containing Sc3+ ions colourless?
   • Sc3+ has a full d sub-shell
   • Sc3+ has no d electrons to be promoted
   • Scandium does not form complexes
   • Sc3+ absorbs all visible light
   Show answer

   Sc3+ has no d electrons to be promoted. Sc3+ is 3d0.

3. Which ion forms colourless aqueous complexes?
   • Cu2+
   • Fe2+
   • Ni2+
   • Cu+
   Show answer

   Cu+. Cu+ is 3d10; the others have partly filled d sub-shells.

4. [Cu(H2O)6]2+ appears blue. Which statement is correct?
   • It absorbs light in the red-orange part of the spectrum
   • It absorbs blue light
   • It emits blue light
   • It absorbs no visible light
   Show answer

   It absorbs light in the red-orange part of the spectrum. We see the colours that are not absorbed.

5. When concentrated HCl is added to aqueous cobalt(II) chloride, the solution turns from pink to blue. What two changes cause this?
   • A change of oxidation number only
   • A change of ligand only
   • A change of ligand and a change of coordination number
   • A change of oxidation number and of ligand
   Show answer

   A change of ligand and a change of coordination number. [Co(H2O)6]2+ (6, octahedral) becomes [CoCl4]2- (4, tetrahedral); Co stays +2.

6. Which equation links the energy gap between split d orbitals to the light absorbed?
   • ΔE = hf
   • E = mc²
   • pV = nRT
   • ΔG = ΔH - TΔS
   Show answer

   ΔE = hf. The photon energy must match the gap: ΔE = hf.

7. An aqueous solution changes from pale green to yellow-brown on standing in air. What has happened?
   • Fe3+ has been reduced to Fe2+
   • The ligands have changed from water to chloride
   • Fe2+ has been oxidised to Fe3+
   • The coordination number has changed from 6 to 4
   Show answer

   Fe2+ has been oxidised to Fe3+. [Fe(H2O)6]2+ is pale green; [Fe(H2O)6]3+ is yellow-brown.

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Sources

• Pearson Edexcel IAL Chemistry (2018) specification
• Pearson Edexcel IAL Chemistry (2018) scheme of work

Last updated 7 November 2026. Spotted a mistake? Tell us.