The magnitude of CFSE (Crystal Field Splitting Energy , `Delta_(0)`) can be related to the configuration of d-orbitals in a coordination entity as

Explanation

When the ligands approach a transition metal ion, the d-orbitals split into two sets, one with lower energy and the other with higher energy. The difference of energy between the two sets of orbitals is called crystal field splitting energy `(Delta_(0)` for octahedral field) if `Delta_(0) lt P` (pairing energy) the fourth electron enters one of the e.g orbitals giving the configuration `t^(3)2g^(e1)g` thus forming high spin complexes Such ligands for which `Delta_(0) lt P` are called weak field ligands if `Delta_(0) gt P` the fourth electron pairs up in one of the `t_(2g)` thereby forming low spin complexes Such ligands for which `Delta_(0) gtP` are called strong field ligands.