Formation of coloured ionsAQA A-Level Chemistry: Revision notes

Transition metal ions can be identified by their colour, for example blue Cu²⁺(aq), pale green Fe²⁺(aq) and pink Co²⁺(aq). White light contains all visible wavelengths. Colour arises when some wavelengths of visible light are absorbed and the remaining wavelengths are transmitted or reflected. The colour seen is the mixture of the light that is not absorbed.

A solution that appears blue has absorbed light at the red and orange end of the spectrum.

In a transition metal complex the d orbitals are not all at the same energy. When light is absorbed, d electrons move from the ground state to an excited state (a higher d energy level). The energy absorbed equals the energy difference:

ΔE = hν = hc/λ

where h is the Planck constant, ν the frequency, c the speed of light and λ the wavelength. A larger ΔE means a higher frequency and shorter wavelength absorbed.

Worked example: light of wavelength 510 nm: ΔE = (6.63 × 10⁻³⁴ × 3.00 × 10⁸) / (510 × 10⁻⁹) = 3.90 × 10⁻¹⁹ J. Multiply by L = 6.02 × 10²³ mol⁻¹ for 235 kJ mol⁻¹.

If the d sub-level is empty (Sc³⁺) or full (Zn²⁺, Cu⁺, [Ar]3d¹⁰), no d–d electron promotion is possible. No visible light is absorbed, so the compounds are white or colourless. Copper(II) sulfate is blue because Cu²⁺ is [Ar]3d⁹.

Changes in oxidation state, co-ordination number and ligand all alter ΔE and so change which wavelengths are absorbed:

• Oxidation state: Fe²⁺(aq) is pale green; Fe³⁺(aq) is yellow-brown
• Co-ordination number and ligand: pink [Co(H₂O)₆]²⁺ becomes blue [CoCl₄]²⁻
• Ligand: pale blue [Cu(H₂O)₆]²⁺ becomes deep blue [Cu(NH₃)₄(H₂O)₂]²⁺

Worked example: [Cu(H₂O)₆]²⁺ absorbs at 800 nm, ΔE = 2.49 × 10⁻¹⁹ J (150 kJ mol⁻¹). The ammine complex absorbs at 600 nm, ΔE = 3.32 × 10⁻¹⁹ J (200 kJ mol⁻¹). The ammonia ligands give a larger ΔE.

The absorption of visible light is used in spectroscopy. A simple colorimeter measures the absorbance of a coloured solution at a chosen wavelength, selected with a filter of the colour that is absorbed most strongly. Absorbance is proportional to concentration.

To find an unknown concentration: make standards of known concentration, measure their absorbances, plot a calibration curve (or find absorbance per mol dm⁻³), then read off the concentration of the sample. If the sample was diluted before measuring, multiply by the dilution factor.

A pale solution can be converted into a more intensely coloured complex, for example with ammonia for Cu²⁺, to improve sensitivity.