Ligands can be arranged in a series in the orders of increasing field strength as given below: Weak field ligands : I−<Br−<S2−<SCN<Cl−<N3−,F− <Urea, OH−<oxalate Strong field ligands O−−<H2O<NCS−<EDTA<Py,NH3< en =SO3−<bipy, Phen<NO2−<CH3− <C6H5−<CN−<CO Such a series is termed as spectrochemical series. It is an experimentally determined series based on the absorption of light by complexes with different ligands.