The absorption of energy or the observation of colour in a complex transition compound depends on the charge of the metal ion and the nature of the ligands attached. The same metal ion with different ligands shows different absorption depending upon the type of ligand. The presence of weak field ligands make the central metal ion to absorb low energies i.e., of higher wavelength. The field strength of ligands can be obtained from spectrochemical series. i.e.

(weak field) I – < Br – < S 2– < Cl – < NO 3 – < F – < OH – < H 2 O < NH 3 < NO 2 – < CN – < CO (strong field)