The Colour of a complex can be altered by any of the factors that can affect the size of the energy gap.
1. Changes in oxidation state:
- If the oxidation state of a transition metal in a complex ion changes, then the colour of the complex ion may also change.
- i.e. Pale green [Fe(H2O)6]^2+(aq) complex >>> Yellow [Fe(H2O)6]^3+(aq) complex
2. Changes in Coordination number:
- Changes in coordination number may also result in a colour change.
- This always involves a change of ligand too.
- i.e. Blue [Cu(H2O)6]^2+(aq) + 4Cl- >>> Yellow [CuCl4]^2-(aq) + 6H2O(aq)
3. Changes in ligand:
- Changing the ligand can cause a colour change even if the oxidation state and coordination number remain the same.
- i.e. Pink [Co(H2O)6]^2+(aq) + 6NH3(aq) >>> Straw/Pale Brown [Co(NH3)6]^2+(aq) + 6H2O(aq)
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