Coordination Compounds

Crystal Field Theory

We have heard of the Valence bond theory, VSEPR theory and even Werner’s theory. But what is the Crystal field theory? Sounds new? Well, it will become familiar once you read this topic. We will cover all of it. But first, we will start with what the theory is all about!

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What is the Crystal Field Theory?

The valence bond theory could not explain the stability of the coordination compounds. It also failed to throw a light on the differences between strong and weak ligands. Therefore, scientists proposed the crystal field theory.

According to this theory, the metal-ligand bond acts as an ionic bond arising purely from the electrostatic interactions between the metal ions and ligands. This theory takes anions as point charges and neutral molecules as dipoles. When the transition metals do not bond to any ligand, their d orbitals degenerate. This means that they have the same amount of energy.

When they start bonding with other ligands, the d orbitals split apart and become non-degenerate. This bonding occurs mainly due to different symmetries of the d orbitals and the inductive effect of the ligands on the electrons. The pattern of the splitting of d orbitals depends on upon the nature of crystal field.

crystal field theory

Splitting of Crystal Fields

In case of an octahedral coordination compound, there are six ligands that surround the metal atom/ion. In these cases, we observe repulsion between the electrons in d orbitals and ligand electrons. This repulsion is more in the case of dx2-y2 and dz2 orbitals. This is because they point towards the axes along the direction of ligand. Hence, their energy is higher as compared to the average energy in spherical crystal field.

While, dxy, dyzand dxz orbitals experience lower repulsions as they are directed between the axes. Hence, these three orbitals possess lower energy than the average energy in spherical crystal field. Thus, we get two energy levels:

  • t2g– set of three orbitals (dxy, dyz and dxz) with lower energy
  • eg – set of two orbitals (dx2-y2 and dz2) with higher energy

Crystal Field Splitting

It is the process of the splitting of degenerate level in the presence of ligand. The difference between the energy of t2g and eg level is denoted by “Δo”. There are some ligands producing strong fields and causing large crystal field splitting. On the other hand, others produce very weak fields.

Thus, the crystal field splitting depends on upon the field produced by the ligand and the charge on the metal ion. We can arrange ligands in order of their field strength as:

I– < Br– < Cl– < SCN– < F– < OH– < C2O42-< H2O <NCS– < EDTA4- < NH3 < en < CN< CO

Solved Example for You

Q: How does the filling of d-orbitals take place, according to the crystal field splitting theory?

Ans: Filling of d-orbitals takes place as follows:

The first three electrons arrange in t2g level as per the Hund’s rule. The fourth electron can either enter into t2g level to result in a configuration of t2g4eg0 or can enter the eg orbital to give a configuration of t2g3eg1.

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