3.2.5.4: Formation of Coloured Ions

• When ligands bond to (metal) ions, some orbitals gain energy, splitting the (3)d-orbitals to new energy levels
• Electrons usally occupy the ground state -> to get to excited state, they must acquire energy = ΔE (the energy gap/difference) in the form of visible light
• The size of ΔE depends on:

  • Oxidation state of metal ion
  • Coordination number & hence type of ligand
  • Shape of complex
• • Frequencies of light absorbed depends on size of ΔE
  • Combination of frequencies creates a complementary colour -> observed with some complexes
  • e.g [Cu(H2O)6]2+ absorbs frequencies that produce red light
  • ∴ complementary colour is blue/cyan (what is reflected/transmitted) -> observed colour is blue
• • Complexes where the (3)d sub-shell/orbital is full
  • no electrons excited to higher levels/excited state
  • complex appears colourless or white
• • Colorimetry is used to measure the conc. of transition metal ions in solution
  • colorimeter is set to zero using blank sample (usually water)
  • sample to be tested is in cuvette
  • colour that filter produces must be the frequency of light absorbed by solution