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 Δo.
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 t2g 3 eg
1 like electronic configuration) or the pairing of the electrons can take place in the t2g orbitals (giving rise to t2g 4 eg 0 like electronic configuration). If the Δo value of a ligand is less than the pairing energy (P), then the electrons enter the eg orbital. On the other hand, if the Δo value of a ligand is more than the pairing energy (P), then the electrons enter the t2g orbital.
