Limitations of Crystal Field Theory

Dhristi JEE 2022-24

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.

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.

FAQs on Limitations of Crystal Field Theory

1. What is the Basis of Crystal Field Theory?

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?

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?

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?

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.

5. How do limitations of Crystal Field Theory explain the bonding in coordination complexes?

The limitations of the Crystal Field Theory account for the covalent interaction between the metal and ligand atoms. Since the theory does not take these interactions into consideration, it leaves a gap in our understanding of coordination complexes. This is where Ligand Field Theory comes in to fill the gap by giving equal importance to the covalent character of bonding in coordination complexes. The coordination complexes are formed due to the covalent character of bonding by sharing electrons. This is why some ligands are stronger than others while some are weaker than others. Crystal Field Theory only took the electrostatic interactions into account and did not consider the covalent interaction between metal and ligand atoms. Later on, Ligand Field Theory was proposed, which gave equal importance to the covalent bonding.

6. What are the assumptions on which Crystal Field Theory is based?

In CFT, some of the important assumptions are:

The ligand molecules always approach perpendicularly to metal ions. The metal ion and ligand occupy a plane. The bond between ligands and metal atoms can be treated as a point contact, i.e., they do not have any covalent character. Ligands always approach the metal ion with their unhybridized p orbital and interact using pure electrostatic interactions. The electrostatic interactions between the ligands and metal ions are additive. One should note that all these assumptions may not be true in reality. However, the theory is successful in explaining most of the coordination complexes.

7. Why couldn't the Crystal Field Theory explain some complexes?

One of the limitations of the Crystal Field Theory is that it cannot explain why some orbitals show large splitting and some show less splitting. It also cannot explain why H2O is a stronger ligand while OH- is weaker than water. These discrepancies can be better explained by Ligand Field Theory which takes into account the covalent character of bonding between metal and ligands. When covalent bonding is taken into consideration, the effects of sharing electrons can be better understood. This is why LFT is a more successful theory than CFT.

8. Is Crystal Field Theory still used?

Yes, the Crystal Field Theory is still used to explain the bonding in coordination complexes. However, it is now supplemented by Ligand Field Theory which takes into account the covalent character of bonding. The combination of these two theories provides a complete understanding of coordination complexes. Students should know that the theory used to explain the bonding in coordination complexes is Ligand Field Theory which gives equal importance to the covalent character of bonding. The Crystal Field Theory does not take into account the covalent interaction between metal and ligand atoms, and this is its main limitation. Vedantu helps students learn chemistry concepts in a fun and interactive way. Our subject experts have created engaging video lessons which will help you understand complex topics like the Limitations of Crystal Field Theory. Our video lessons are accompanied by practice questions and solutions so that you can test your understanding and get better grades in your exams.

9. What is the role of the metal and ligand in a coordination complex?

The role of the ligand is to provide electrons, while the role of donor atoms or groups on metal ions is to accept them. This helps in the formation of coordinate covalent bonds. The geometry of a coordination complex can be considered as octahedral, square pyramidal, tetrahedral or square planar, do. The ligands which are strong and those weak depend on the number of electrons donated and accepted, i.e., whether it is a high or low spin complex. Students should also know that a high spin complex has a more stable octahedral geometry while a low spin complex is tetrahedral in geometry.

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