Ligand Field Theory (LFT) explains the spectrochemical series by showing how different ligands cause different amounts of splitting of the metal’s d-orbitals. Here’s a explanation:
1. What is the Spectrochemical Series?
- It’s a list of ligands arranged according to how strongly they split the metal d-orbitals.
- Weak-field ligands → small splitting
- Strong-field ligands → large splitting
2. How LFT Explains It
- LFT considers both σ-donation and π-interactions between ligands and the metal:
- σ-Donation:
- All ligands donate electrons to the metal via sigma bonds.
- Stronger σ-donors increase splitting slightly.
- π-Interactions:
- π-acceptor ligands (like CO, CN⁻) pull electron density from the metal → increase splitting.
- π-donor ligands (like halides) push electrons toward the metal → decrease splitting.
- The combination of these effects determines the ligand’s field strength in the spectrochemical series.
3. Resulting Order
- Weak-field ligands (small splitting, high-spin) → I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O
- Strong-field ligands (large splitting, low-spin) → NH₃ < en < CN⁻ < CO
In short
- LFT explains the spectrochemical series because different ligands affect d-orbital splitting differently through σ and π bonding.
- This splitting determines spin state, color, and reactivity of the complex.