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 (eg and t2g) is called the crystal-field splitting energy. It is denoted by ∆_o.

Fig. d-orbital splitting in an octahedral crystal field.

The formation of complex depend on the crystal field splitting, ∆_o and pairing energy (P).

i)If ∆_o < P, the fourth electron enters one of the eg orbitals giving theconfiguration t_2g³. Ligands for which ∆_o < P are known as weak field ligands and form high spin complexes.

ii) If ∆_o > P, it becomes more energetically favourable for the fourth electron to occupy a t_2g orbital with configuration t^_2g4 e_g⁰. Ligands which produce this effect are known as strong field ligands and form low spin complexes.