HL
Complex Ions & Color
Unlocking Crystal Field Theory to explain the colors of d-block complexes.
1. Crystal Field Theory (CFT)
The Mechanism of Color
- Ligand Approach: A ligand (Lewis Base) forms a coordinate bond with the metal ion, repelling the d-orbitals.
- Splitting: The 5 degenerate d-orbitals split into two energy
levels:
- Lower energy set (\(t_{2g}\))
- Higher energy set (\(e_g\))
- Electron Promotion: Light energy (\(h\nu\)) is absorbed to promote an electron from lower → higher. This is a d-d transition.
- Observed Color: The complementary color to the wavelength absorbed.
2. Factors Affecting Splitting Energy (\(\Delta E\))
Identity of Metal
Charge: Higher charge = more interaction with ligands = Larger Splitting.
Spectrochemical Series
Ligands ordered by splitting strength:
\(I^- < Br^- < Cl^- < F^- < OH^- < H_2O < NH_3 < CN^- < CO\)
Strong Field Ligands (like CN⁻) cause large splitting, absorbing violet/blue light (appearing yellow/red).