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AHL Extension3.1.6
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 ()
- Higher energy set ()
- Electron Promotion: Light energy () is absorbed to promote an electron from the lower to the higher set. This is a d-d transition.
- Observed Color: The complementary color to the wavelength absorbed.
2. Factors Affecting Splitting Energy ()
Identity of Metal
Charge: Higher charge = more interaction with ligands = Larger Splitting.
Spectrochemical Series
Ligands are ordered by splitting strength.
Strong Field Ligands (like CN⁻) cause large splitting, absorbing violet/blue light (appearing yellow/red).