Crystal Field Theory
In crystal field theory, we assume the ligands to be the point charges and there is interaction between the electrons of the ligands and the electrons of the central metal atom or ion. The five d-orbitals in an isolated gaseous metal atom or ion are degenerate. The degeneracy is maintained if an spherically symmetrical negative field surrounds the metal atom/ion. However, when ligands appraoch the central metal atom/ion, the field created is not exactly spherically symmetrical  and the degeneracy of the d-orbitals is lifted. It results in the splitting of d-orbitals and the pattern of spitting depends upon the nature of the crystal field. This splitting of d-orbitals energies and its effects, form the basis of the crystal field treatment.
Ligands that cause large degree of crystals field splitting are termed as strond field ligands. Ligands that cause only a small degree of crystal field splitting are termed as weak field splittings. The common ligands can be arranaged in ascending order of crystal field splitting energy. The order remains practically constant for different metals and this series is called the spectrochemical series.