[Chemistry Class Notes] Limitations of Crystal Field Theory Pdf for Exam

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.

What is Crystal Field Theory?

Crystal Field Theory is a theory that explains the bonding in coordination complexes. In this theory, the atoms are treated as hard spheres, and the 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. One should not think that the Crystal Field Theory is wrong. It is also one of the accepted theories and has been successfully used to explain many facts about coordination compounds. But, when electron-electron repulsion (interaction between filled orbitals) was taken into account in computations, new forms of bonding were discovered.

Why Do We Need a Different Explanation For Bonding In Coordination Compounds?

The ligands present around the central metal atom are not only negatively charged ions but also neutral molecules. Thus, they attract the positive charge of the central atom with both electrostatic and covalent attraction. This poses a problem for understanding their bonding through just considering electrostatics. One can understand the formation of metal-ligand bonds by considering both electrostatics and covalent interactions.

Formation of Metal-Ligand bonds

In a coordination complex, the central metal atom forms coordinate bonds with the surrounding ligand. When a neutral molecule approaches the positive metal ion, the electrostatic interaction causes changes in the energy levels of the metal ion. By lowering energy, some electrons start moving to lower energy orbitals. This creates a partially filled orbital on the metal atom. The neutral molecule now has an empty orbital that can interact with this partially filled orbital on the metal atom. This can be further understood by the diagram given below:

This is how the bond forms in coordination complexes. The above explanation gives a clear understanding of the bonding in coordination complexes. Though the Crystal Field Theory is successful in explaining most of the cases, it fails to take into account the fact that the ligands do not attract the positive charge of the central atom with just electrostatic forces. The above explanation is known as ‘Valence Bond 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 is always considered only d orbitals. The contribution of s and p blocks was never taken into consideration.

• It treated the atoms as hard spheres, and interaction between them was considered 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 behavior of some complexes and 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 considered the ionic character.

• 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.