Limitations of Crystal Field Theory

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Introduction

Crystal Field Theory is one of the most accepted theories proposed for explaining the bonding in coordination complexes. It was proposed by Hans Bethe. It treated the atoms as hard spheres and interaction between them is purely electrostatic. The central metal atom has a positive charge and the surrounding ligands have a negative charge. When this negatively charged ion approaches towards the positive metal ion, the electrostatic attraction causes changes in the energy levels of the metal ion and bonds to form in the end. Crystal Field Theory was successful in explaining the bonding in most of the complexes. But it had some limitations too. In this article, we will discuss the limitations of the Crystal field theory.


Limitations of Crystal Field Theory

Theories are always based upon assumptions. These assumptions may fit perfectly with experimental data. However, there are some limitations of CFT too; which are discussed below:

  • This theory always took only d orbitals into consideration. Contribution of s and p block was never taken into consideration.

  • It treated the atoms as hard spheres and interaction between them was considered totally electrostatic. But it cannot be true in reality.

  • This theory did not give any importance to the orbitals of the ligands. It only talked about the central metal ion.

  • This theory could not explain the behaviour of some complexes about why some orbitals show large splitting and some show less splitting.

  • It cannot explain why H2O is a stronger ligand while OH-  is weaker than water.

  • One of the main drawbacks is that CFT does take care of covalent character between metal and ligands. It only took the ionic character into consideration.

  • All the consequences and effects that are present due to the covalent character are not explained by Crystal Field Theory.

So, these are the limitations of crystal field theory. To overcome these limitations, Ligand Field Theory (LFT) was proposed later. LFT gave equal importance to the ligand orbitals.

FAQs (Frequently Asked Questions)

1:  What is the Basis of Crystal Field Theory?

Answer: This theory was developed to overcome the drawbacks of Valence Bond Theory. It focussed on the properties and nature of bonding between ligands and central metal atoms. Interaction between metal ions and ligands is the basis of this theory.

2: What are the Applications of Crystal Field Splitting?

Answer: It helped us in determining the colour, shape, spectra, chemical and physical properties of various complexes. These properties are useful at a commercial level to use different complexes as per the need. In the dye industry, knowing the colour of the compound helped in the creation of different pigments.

3: What are the Factors affecting Crystal Field Splitting?

Answer: Crystal Field Splitting considers the interaction between metal ion and ligand during bond formation. So, the nature of metal ion as well as a ligand, size of metal ion, charge, number of surrounding ligands, the coordination number of metal ion are the factors affecting Crystal Field Splitting.

4: Why is CFT Better than VBT?

Answer: Valence Bond Theory (VBT) explained the mixing of orbitals during bond formation. It mainly used hybridisation concepts for the explanation. While Crystal Field Theory explains how orbitals split when ligands approach the metal ions, VBT could not explain the magnetic behaviour satisfactorily. It could not explain the formation of outer orbital and inner orbital complex formation. But, CFT explained this all.