Question

Discuss the salient features of crystal field theory.

Answer 1

1. In coordination complexes, the central metal ion is surrounded by ligands, which may be negatively charged ions (e.g., F⁻, Cl⁻, CN⁻) or neutral molecules with lone pairs (e.g., NH₃, H₂O, en, gly).
2. In neutral ligands, the most electronegative atom directs its lone pair toward the metal ion to form the coordinate bond.
3. Both metal ions and ligands are treated as point charges, and the interaction between them is considered purely electrostatic.
4. When ligands approach the metal ion, repulsion occurs between the electrons of the ligands and the d-electrons of the metal ion.
5. This repulsion removes the degeneracy of the five d-orbitals, splitting them into two energy levels: t₂g (lower energy) and e_n (higher energy), depending on their orientation.
6. The orbitals pointing directly toward ligands experience greater repulsion and become higher in energy, while those between ligand directions are less affected.
7. The difference in energy between the two sets of orbitals is known as crystal field splitting energy (Δ), commonly measured as 10 Dq.
8. The amount of splitting depends on the geometry of the complex (e.g., octahedral or tetrahedral) and the nature and number of ligands.
9. Electrons fill the split d-orbitals according to Hund’s rule, occupying the lower-energy orbitals first to minimize repulsion.
10. The stability of a complex is explained by the crystal field stabilisation energy (CFSE), which arises from the arrangement of electrons in the split orbitals.
11. The energy gained due to this arrangement is called Crystal Field Stabilisation Energy (CFSE).
12. The greater the CFSE, the more stable the complex is.