9.18 What is crystal field splitting energy? How does the magnitude of o decide the actual configuration of d orbitals in a coordination entity?

Step 1:

The degenerate d-orbitals (in a spherical field environment) split into two levels i.e., eg and t2g in the presence of ligands.
The splitting of the degenerate levels due to the presence of ligands is called the crystal-field splitting while the energy difference between the two levels (eg and t2g) is called the crystal-field splitting energy. It is denoted by ∆o.

Step 2:

After the orbitals have split, the filling of the electrons takes place. After 1 electron (each) has been filled in the three t2g orbitals, the filling of the fourth electron takes place in two ways.

It can enter the eg orbital (giving rise to t2g3 eg1 like electronic configuration) or the pairing of the electrons can take place in the t2g orbitals (giving rise to t2g4 eg0 like electronic configuration).

If the ∆o value of a ligand is less than the pairing energy (P), then the electrons enter the eg orbital. On the other hand, if the ∆o value of a ligand is more than the pairing energy (P), then the electrons enter the t2g orbital.