Crystal Field Theory
d-Orbital splitting in coordination complexes
Octahedral Splitting
Δo = crystal field splitting energy
eg orbitals (dz², dx²-y²) higher energy
t2g orbitals (dxy, dxz, dyz) lower energy
Ligands on axes repel eg orbitals more than t2g.
Tetrahedral Splitting
Δt ≈ (4/9) Δo
Reverse order: e orbitals lower, t₂ orbitals higher
Spectrochemical Series
Ligands ranked by Δ (weak to strong field):
I⁻ < Br⁻ < Cl⁻ < F⁻ < H₂O < NH₃ < en < NO₂⁻ < CN⁻ < CO
High-Spin vs Low-Spin
- Weak field (small Δ): electrons prefer to remain unpaired → high-spin
- Strong field (large Δ): pairing energy < Δ → electrons pair → low-spin
- Example: [Fe(H₂O)₆]³⁺ high-spin (5 unpaired); [Fe(CN)₆]³⁻ low-spin (1 unpaired)
Crystal Field Stabilization Energy (CFSE)
Energy gained by placing electrons in lower orbitals:
CFSE = (number in t2g) × (-0.4 Δo) + (number in eg) × (+0.6 Δo)