Crystal Field Theory

What is Crystal Field Theory (CFT)? Discuss its assumptions, Limitations and Advantages.

What is Crystal Field Theory (CFT)? Discuss its assumptions, Limitations and Advantages.


Crystal Field Theory

Crystal Field Theory (CFT) was developed by Hans Bethe and John Hasbrouck van Vleck in 1929. CFT considers the interaction of metal ion and ligand as a purely electrostatic phenomenon where the ligands are considered as point charges in case of anions or dipoles in case of neutral molecules in the vicinity of the atomic orbitals of the central atom.
Crystal field theory describes the breaking of orbital degeneracy in transition metal complexes due to the presence of ligands. Crystal field theory qualitatively describes the strength of the metal-ligand bonds and based on the strength of the metal-ligand bonds, the energy of the system is altered. This may lead to a change in magnetic properties as well as the color.


Postulates or Assumptions of Crystal Field Theory

1. According to the crystal field theory, the metal ion is surrounded by an electric field created by the ligands.
2. In a complex, the attraction between the metal atom and the ligand is purely electrostatic. The metal ion is targeted by the negative end of the dipole of the neutral molecule ligand.
3. The transition metal or metal ion have the same charge as the oxidation state.
4. A specified number of ligands surround the transition metal atom or ion, may be negative ions or neutral molecules with lone pairs of electrons.
5. The ligands act as point charges that generate an electric field. The energy of the orbitals on the metal atom or ions is changed by this electric field.

6. The electrons on the central metal ion occupy the d-orbitals as far away as possible from the direction of approach of the ligand due to the repulsive attraction between the central metal ion and the ligand.
7. The metal orbital and the ligand orbital have no interaction.
8. All orbitals have the same energy in an isolated metal atom or ion, i.e., all five d orbitals (dxy, dxz, dyz, dx2–y2 and dz2) are degenerate.
9. The d-orbitals remain degenerate when the metal atom or metal ion is surrounded by a spherically symmetrical field of negative charges. The repulsion between the field and the electron on the metal atom or ion, however, raises the energy of the orbitals.
10. The d-orbitals are influenced differently in most transition metal complexes, and their degeneracy is lost due to the field produced by the unsymmetrical ligand.


Limitations of Crystal Field Theory

The limitations or drawbacks of crystal field theory are given below-
1. The assumption that the interaction between metal-ligand is purely electrostatic can not be said to be very realistic.
2. CFT only takes d-orbitals of central atom in to account. The s and p orbitals are not considered.
3. CFT fails to explain the behaviour of certain metals which cause large splitting while others show small splitting.
For example, CFT does not explain why H2O is a stronger ligand as compared to OH.
4. CFT fails to explain the possibility of having p bonding because P bonding is found in many complexes.
5. CFT gives no significance to the orbitals of the ligands.
Hence,it can not explain any properties related to ligand orbitals and their interaction with metal orbitals.


Advantages of Crystal Field Theory

Followings are some advantages of crystal field theory-
This theory can be used to describe the stability of complexes.
The greater the crystal field splitting energy, the greater the stability.
Colour and spectra of the complexes can be explained using this theory.
This theory explains the magnetic properties of complexes.

Splitting of d-Orbitals or Crystal Field Splitting