A-Level Chemistry May/June 2025 Q6(a)(ii): Explain why a solution containing complex ion A and a solution containing complex ion B…

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  The colour of transition metal complexes is due to the splitting of the d-orbitals into two different energy levels by the ligands.

  For complex ion A and complex ion B, the degree of d-orbital splitting is different. This results in a different energy gap, ΔE, between the lower and higher energy d-orbitals for each complex.

  Consequently, the two complexes absorb different frequencies (or wavelengths) of visible light, as the energy of the absorbed photon must be equal to ΔE. The colour observed is the complementary colour of the light absorbed, so because different frequencies are absorbed, the solutions appear as different colours.

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  How the marks are awarded

  • M1 — Stating that the energy gap between the split d-orbitals, ΔE, is different for the two complex ions.
  • M2 — Linking the different energy gap to the absorption of different frequencies or wavelengths of visible light.

  Common mistakes

  • Simply stating 'they absorb different colours' which is too vague; the answer must refer to frequency or wavelength of light.
  • Forgetting to mention the difference in the d-orbital splitting energy (ΔE) as the fundamental reason for the colour difference.
  • Confusing the absorbed colour with the observed (transmitted) colour.
  • Incorrectly describing the process as being due to the emission of light from the complex.

  Examiner tip: Always connect the observed colour of a transition metal complex to the specific frequency of light absorbed, which in turn is determined by the magnitude of the d-orbital splitting energy, ΔE.

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