For d4 ions, two possible patterns of electron distribution arise:
(i) If Δo < P, the fourth electron enters one of the eg orbitals giving the configuration t32g e1g . Ligands for which Δo < P are known as weak field ligands and form high spin complexes.
(ii) If Δo > P, it becomes more energetically favourable for the fourth electron to occupy a t2g orbital with configuration t42g e0 g. Ligands which produce this effect are known as strong field ligands and form low spin complexes.