What are the postulates of crystal field theory ?
The postulates of crystal field theory are
- Ionic ligands such as Cl-, OH-, CN- are regarded as negative point charges (or simply point charges) and the neutral ligands such as H2O, NH3, py are regarded as point dipole (or simply dipoles) because these ligands are dipolar. If the ligand is neutral molecules such as H23O, NH3 the negative end of the dipole is directed towards the metal ion.
- Metal-ligand bond is not covalent, i.e., there is no overlapping of orbitals. Instead, the bonding in complexes is purely electrostatic in nature. In complexes, two types of electrostatic forces come into account. One is the attraction between the metal cation and the negatively charged ligand or the negative end of the polar ligand (i.e., dipole). The second type of electrostatic interaction is the repulsion between the lone pairs of electrons on the ligands and the electrons in the d-orbitals of the metal cation and the repulsion between nuclei of the metal cation and the ligands but to a small extent. Another repulsion also comes into account that occurs among the ligands.
- The five d-orbitals in a free metal ion are degenerate (i.e., have the same energy). When a complex is formed, the electrostatic field of the ligands destroys the degeneracy of these d-orbitals, i.e., these orbitals now have different energies. The orbitals lying in the direction of the ligands are raised in energy more than those lying away from the ligands because of the repulsion between the d-orbitals and the ligands.