When ligands approach a transition metal ion, the ‘rf orbital splits into two sets one with lower energy and the other with higher energy. The difference in energy between two sets (t2g and eg) of orbitals is called crystal field splitting energy Δo for an octahedral field.
If Δo < p (pairing energy), the fourth electron enters one of the eg orbitals giving the configuration t2g3 eg1, thereby forming high spin complexes. Such ligands for which Δo < p are called weak field ligands.
If Δo > p, the fourth electron pair up in one of the t2g orbital giving the configuration t2g4 eg0, thus forming low spin complexes. Such ligands for which Δo > p are called strong field ligands.
