Question

What are the limitations of crystal field theory?

Answer 1

Hint: CFT is a theory that is used to describe the stability of the metal complexes. It revolves around the loss of orbital degeneracy because of the uneven charge distribution due to the approach of the ligands towards the metal ion in specific directions to give the required geometry.

Complete step by step answer:
Coordination compounds are formed by cations and the ligands coordinated with them. These ligands remove the orbital degeneracy in transition metal ions as they approach it. This phenomenon is described as crystal field theory developed by Hans Bethe and John Hasbrouck van Vleck. It describes the stabilization of the metal-ligand complexes qualitatively. This change in orbital degeneracy is further used to describe other phenomena such as magnetic properties and the colour of the complex.
There are very few coordination compounds of large cations of low charge, such as K⁺ and Na⁺. However that is not the case for transition metal cations since they have electrons in d orbitals that are not spherically symmetric in nature. Their shape and occupation becomes a determining factor of how they are going to split up for a particular symmetry.
For a particular transition metal ion, the five d-orbitals are degenerate. When ligands approach the metal ion in order to form a complex, some experience more repulsion from the d-orbital electrons than others based on the direction along which they are approaching in order to form a complex with a particular geometry. This leads to a split due to the electrostatic environment which is not uniform.
Let us consider a complex with octahedral geometry. Ligands approach the metal ion along the x,y and z direction. Due to this approach, the electrons in the dz² and dx²−y² orbitals (which lie along the direction along which the ligands are approaching) experience more repulsion than other orbitals. This leads to a split between the d-orbitals which is known as crystal field splitting. “For octahedral complexes, crystal field splitting is denoted by Δo (or Δoct )”. The dz² and dx²−y² orbitals are raised in energy while the dxy , dxz and dyz orbitals are lowered in energy with respect to the initial energy level possessed by the degenerate d orbitals. This leads to more stability.
CFT is applicable for all geometries as long as the metal cation possesses the d electrons.
Limitations of CFT:
It does not take into account the covalent character in metal-ligand bonding in the complexes. According to CFT, the metal cation and the ligands are point charges and their interactions are purely electrostatic in nature which is not true. Also it does not give any satisfactory explanation regarding the relative strengths of different ligands, why a particular ligand is a strong field ligand while the other is a weak field ligand.
It fails to explain the ${ \Pi }$ bonding and back-bonding between the metal atom and the ligand and its effects on CFSE. It does not take into account the s and p orbitals of the central metal atom.
It does not take into account the orbitals of the ligands.

Note: Many strong experimental evidences have proven that the metal complexes have some covalent character. These include the Nephelauxetic effect; Land’s splitting factor, electron spin resonance spectra and nuclear magnetic resonance spectra, nuclear quadrupole resonance, Kramers-Anderson super exchange.