metal complexes are formed when a metal ion bonds with electron pair donors called ligands. metal complexes are known to exhibit different colors depending on the metal center and the ligands involved. as the reaction reaches equilibrium, one can write the formation constant or stability constant whose values can be determined by substituting the actual equilibrium concentrations of the species involved. a higher value means that the complex is stable and more likely to be formed. the stability of a metal complex depends on how tightly bound the ligands are to the metal ion. ligands are arranged based on the strength of their interactions with metal centers in a list called the spectrochemical series. most of the metal complexes form colored solutions. the color that we perceive is complementary to the wavelength of light absorbed by the complex. weak field ligands tend to absorb radiation of longer wavelengths while strong field ligands absorb light of shorter wavelengths.