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Crystal Field Theory and Its Limitations

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Limitation of Crystal Field Theory: An Introduction

In 1930, Hans Bethe, a physicist, proposed a theory called crystal field theory which overruled the valence bond theory, the already existing theory to explain bonding in metal complexes. Crystal field theory is often abbreviated as CFT. According to crystal field theory, the interaction between metal and ligands is purely electrostatic attraction. This means their metal atom in the centre is considered as a positively charged hard sphere and the surrounding ligands are negatively charged species either an anion or neutral species with lone pairs of electrons.

When these ligands approach the metal, the electron cloud of the ligand can disturb the degeneracy of d orbitals of metal atoms and eventually forms bonds with the metal. By applying this theory, scientists explained the bonding in most of the complexes. Along with the advantages of CFT, we have to account for its limitations also. The two limitations of crystal field theory are it ignores contributions of s and p orbital of the metal and it says nothing about the orbitals of ligands. It has certain number of limitations like this. This article gives a thorough understanding of the limitations of CFT. Limitations of crystal field theory pdf can be downloaded for more information.

Nature of Bond in Coordination Complexes: Need of a Special Theory

The metal atom in the centre is electron deficient, which means it has vacant orbitals to accommodate electrons. Ligands are electron-rich species. They are either negatively charged anions or neutral species which carry lone pairs of electrons. Hence, they attract positively charged metal atoms both electrostatically and covalently. This makes it difficult to understand the nature of bonds purely electrostatically. By considering both the covalent and electrostatic nature of the bond, one can understand the nature of the bond. Hence, special theories should be implemented for this.

What is Crystal Field Theory?

Crystal field theory considers the central metal atom in a coordination complex as positively charged hard spheres. The metal atom has vacant orbitals to accommodate more electrons and hence is considered electron deficient. The groups surrounding the metal atom are called ligands. This is the anionic part, hence it has a negatively charged electron cloud.

When this electron cloud comes close to a metal atom, there should be an electrostatic attraction between metal and ligand. The d or f orbital of central metal is degenerate, which means all the orbitals have the same energy level. This degeneracy of the orbital of the metal will lose when the electron cloud of the metal approaches. And these ligands form bonds with the metal atom.

CFT is used to explain many of the properties of coordination complexes like the stability of the complex, colour, magnetic properties, and spin of the complexes. Also, this is the base for explaining many spectrums of the complex.

What Are The Limitations of Crystal Field Theory

Crystal field theory explains most of the properties of the coordination complexes and the bonding between metal and ligand in the complex. But it has certain limitations too. The major limitations of this theory are given below.

  • Crystal field theory considers only the d orbital of a metal atom. But in some cases, the contribution of s and p orbitals should also be taken into account.

  • According to crystal field theory, the bonding in metal and ligand is purely electrostatic attraction. But it is not actually true.

  • It does not consider the covalent nature of the bond between the ligand and the central metal.

  • Crystal field theory says nothing about the orbitals of ligands. It only focuses on the metal orbital.

  • It does not account for why some ligands split the d orbitals greatly and some ligands split the d orbitals shortly.

  • It can not explain why H2O is a strong field ligand and why OH- is a weak field ligand.

  • It can not explain all the effects and consequences due to the covalent nature of the bond.

Hence, to overcome these limitations, a new theory was proposed known as ligand field theory. This theory gives equal importance to the metal and ligand orbitals.

Limitations of Crystal Field Theory with Examples

One of the main limitations of crystal field theory is that it can not explain why certain ligands are strong field ligands and some are weak field ligands. For example water is a strong field ligand. It splits metal orbitals to a greater extend than hydroxyl ion. Crystal field theory could not explain the reasob for such variations.

Key Features

  • Crystal field theory deals with the electrostatic nature of bonding between a ligand and central metal atom.

  • It explains a number of properties of the complexes like stability, colour, spin, reactivity etc.

  • It has certain limitations.

  • The major limitations of this theory are it does not consider the covalent nature of the bond, ligand orbitals, and field effect of ligands.

  • To overcome the limitations of crystal field theory, ligand field theory was proposed.

Last updated date: 23rd Sep 2023
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FAQs on Crystal Field Theory and Its Limitations

1. How does CFT account for the colour of the complex?

According to CFT, the d orbital of metal splits into different energy levels under the influence of ligand. When white light falls on the complex, there occurs a d-d transition of electrons from the lower energy d orbital to the upper energy level. During this transition, the electron absorbs light of a particular wavelength and the rest of the light is reflected.

2. What is the main difference between CFT and VBT?

VBT is based on the mixing up of the atomic orbital of the central metal whereas CFT is the splitting of orbitals under the influence of ligands into higher and lower energy levels.

3. What properties of coordination complexes can be predictable by using CFT?

CFT can explain the stability, reactivity, colour, magnetic property, spinel structure, and spectra of complexes.

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.

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.

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.