The crystal field splitting, Δo, depends upon the field produced bythe ligand and charge on the metal ion. Some ligands are able to producestrong fields in which, the splitting will be large whereas others produceweak fields and consequently result in small splitting of d-orbitals. Ingeneral, ligands can be arranged in a series in the order of increasing fieldstrength as given below :
Such a series is termed as spectrochemical series.
If Δo< P, the fourth electron enters one of the egorbitals giving theconfiguration t23geg1. Ligands for which Δo < P are known as weak fieldligands and form high spin complexes. If Δo > P, it becomes moreenergetically favourable for the fourth electron to occupy a t2g orbitalwith configuration t2g4eg0. Ligands which produce this effect are known asstrong field ligands and form low spin complexes.