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., ${e}_{g}$ and ${t}_{2g}$ 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 (${e}_{g}$ and ${t}_{2g}$) 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 ${t}_{2g}$ orbitals, the filling of the fourth electron takes place in two ways.

It can enter the ${e}_{g}$ orbital (giving rise to  like electronic configuration) or the pairing of the electrons can take place in the ${t}_{2g}$ orbitals (giving rise to  like electronic configuration).

If the ∆o value of a ligand is less than the pairing energy (P), then the electrons enter the ${e}_{g}$ orbital. On the other hand, if the ∆o value of a ligand is more than the pairing energy (P), then the electrons enter the ${t}_{2g}$ orbital.