Colour in transition metal complexesEdexcel International A Level Chemistry: Revision notes

Aqueous solutions of transition metal ions are usually coloured. The colour comes from the d electrons.

White light contains all visible frequencies. A d electron can absorb light and be promoted to a higher-energy d orbital. The energy absorbed is

ΔE = hν

where h is Planck's constant and ν is the frequency. Only light of that frequency is absorbed. The remaining frequencies are transmitted, and the solution has the colour of the light that is left.

For example, [Cu(H₂O)₆]²⁺ absorbs red and orange light and transmits blue.

In a free metal ion the five 3d orbitals have the same energy. When ligands bond to the ion, their lone pairs repel electrons in some d orbitals more than others, so the d orbitals split into two groups at different energies.

The energy gap ΔE is small enough to correspond to visible light. Its size depends on:

• the metal ion and its oxidation number
• the ligand
• the coordination number and shape of the complex

A larger ΔE means light of a higher frequency (shorter wavelength) is absorbed.

An electron can be promoted only if there is a vacant d orbital to go into and a d electron to move.

• Sc³⁺ is [Ar] with no d electrons, so nothing can be promoted. Ti⁴⁺ is the same.
• Zn²⁺ is [Ar] 3d¹⁰ and Cu⁺ is [Ar] 3d¹⁰. The 3d sub-shell is full, so there is no vacant d orbital.

No visible light is absorbed, so all of it is transmitted and these ions are colourless.

Ions with partly filled 3d sub-shells, such as Cu²⁺ (3d⁹), Fe²⁺ (3d⁶) and Cr³⁺ (3d³), are coloured.

A colour change arises from a change in ΔE. It can result from a change in:

• Oxidation number: Fe²⁺(aq) is pale green and Fe³⁺(aq) is yellow.
• Ligand: [Cu(H₂O)₆]²⁺ (pale blue) becomes [Cu(NH₃)₄(H₂O)₂]²⁺ (deep blue); Ni²⁺(aq) (green) becomes [Ni(NH₃)₆]²⁺ (blue).
• Coordination number (and ligand): [Cu(H₂O)₆]²⁺ + 4Cl⁻ → [CuCl₄]²⁻ + 6H₂O, blue to yellow-green; [Co(H₂O)₆]²⁺ (pink) becomes [CoCl₄]²⁻ (blue).

You should be able to write the ligand-exchange equations and name the change that has occurred.

Use these steps for a 4–6 mark answer.

1. State the electronic configuration of the ion, e.g. Ti³⁺ is [Ar] 3d¹.
2. The ligands split the d orbitals into two energy levels.
3. A d electron is promoted, absorbing light of frequency ν where ΔE = hν.
4. The unabsorbed frequencies are transmitted, giving the colour seen.

For a colourless ion, say there is no d electron (3d⁰) or no vacancy (3d¹⁰), so no visible light is absorbed.

For a colour change, name what changed (oxidation number, ligand or coordination number) and say that ΔE changed.