Answer

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 Δ_o.

 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 eg orbital (giving rise to t_2g³ e_g¹ like electronic configuration) or the pairing of the electrons can take place in the t_2g orbitals (giving rise to t_2g⁴ e_g⁰ like electronic configuration). If the Δ_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 Δ_o value of a ligand is more than the pairing energy (P), then the electrons enter the t_2g orbital.