The strong ligands have higher splitting power of d orbitals of the central metal ion, whereas weak ligand has relatively lower splitting power of d orbitals of the central metal ion. The energy difference between t_2g and e_g sets of d orbitals is CFSE. The strength of the ligands depend on the magnitude of Δ . Strong ligands have larger value of CFSE and in case of weak ligands the CFSE values are smaller. The common ligands can be arranged in a series in the order of their decreasing field strength, as follows.

I^_<Br ^-< SCN^-< Cl^-< F^-< OH^-< C₂O₄^2-< H₂O< NCS^-< py< NH₃< en< bipy< o-phen< NO₂^-< CN^-< CO.

This series depends on the power of splitting the d orbitals and is called spectrochemical series, The order of field strength of the common ligands neither depends on the geometry of the complex the nature of central metal ion.