Explain colour from d–d transitions and absorbed light
Relate ligand-field energy splitting to photon frequency and to the complementary colour observed.
Section 28 · Chemistry of transition elements
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 28. Use the first-row transition elements titanium to copper and the species explicitly named in the syllabus. Where redox potentials or stability constants are needed, use the values supplied in the question. Colour descriptions can depend on conditions and concentration. Written practice is self-checked
Before you begin
- Use electron configurations, oxidation numbers, ionic equations, equilibrium expressions and the AS redox and bonding models.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What happens when a d electron absorbs a photon matching ΔE?
What you will learn
- Explain colour through promotion of an electron between non-degenerate d orbitals
- Relate ΔE to the frequency of absorbed light
- Infer that the observed colour is complementary to the absorbed colour
An electron can absorb light during promotion
When a complex absorbs light with energy matching ΔE, an electron can be promoted from a lower to a higher d-orbital energy level. The syllabus links the splitting of d orbitals with the frequency of the light absorbed.
Photon energy is related to frequency by E = hν. A larger ΔE requires higher-frequency, shorter-wavelength light; a smaller ΔE corresponds to lower-frequency, longer-wavelength light. Use this relation qualitatively unless data are supplied.
The observed colour is complementary to absorbed light
A solution appears the colour of the visible light that is transmitted or reflected rather than absorbed. Therefore, the observed colour is complementary to the absorbed colour in the syllabus model.
Different ligands can change ΔE, so they can change which frequency is absorbed and hence the complementary colour seen. Do not equate the absorbed colour directly with the colour named for the solution.
Pause and explain
If a complex absorbs green light, how is its observed colour related?
Worked example
A complex absorbs orange light. What can be said qualitatively about the colour observed?
Show the worked solution
- Some orange light is removed from the transmitted light.
- The observed colour is complementary to orange.
- Avoid naming a precise shade unless the colour chart or data are supplied.
Answer The complex appears a complementary colour to orange because orange light is absorbed; the observed colour is not the absorbed colour itself.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
State the relationship between ΔE and the frequency of light absorbed.
Give me a hint
Use photon energy E = hν.
Compare my reasoning
- The absorbed photon must supply the transition energy.
- E = hν.
- A larger ΔE means a higher required frequency.
ΔE = hν for the absorbed photon; a larger ΔE corresponds to a higher frequency of absorbed light.
Look for these in your work
- Energy match stated
- Equation used
- Frequency direction correct
A complex appears blue. Which colour relation should a student state about the absorbed light?
Give me a hint
Observed and absorbed colours are complementary in this model.
Compare my reasoning
- Blue is the observed colour.
- The absorbed visible colour is complementary to blue.
- Do not claim the solution absorbs blue simply because it appears blue.
The complex appears blue, so the absorbed visible light is complementary to blue, not necessarily blue itself.
Look for these in your work
- Observed colour identified
- Complementary relation stated
- Absorption distinguished
Two complexes of the same metal have different ligands and show different colours. Give the syllabus-level chain connecting ligand identity to colour.
Give me a hint
Link ligand field, ΔE, frequency absorbed and complementary colour.
Compare my reasoning
- Different ligands can change the d-orbital energy gap ΔE.
- A different ΔE changes the frequency absorbed.
- The complementary transmitted/reflected colour can therefore differ.
Different ligands alter ΔE; the electron transition then absorbs a different light frequency, so the complementary colour observed can change.
Look for these in your work
- Ligand effect on ΔE
- Absorbed frequency linked
- Observed colour derived
Common mistakes
- Saying the solution appears the colour it absorbs
- Claiming all transition-metal ions are coloured
- Ignoring d-orbital splitting
- Giving an exact wavelength without a supplied ΔE or spectrum
What can you explain now?
What happens when a d electron absorbs a photon matching ΔE?
Compare with the explanation
It can be promoted to the higher-energy d set
The absorbed photon supplies the energy gap.
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Make it stick
How does a ligand-dependent ΔE lead from absorption of light to the colour a complex appears?
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