ESR spectra of copper (II) was measured in 33 copper complexes which were synthesized in this laboratory and the bonding parameter α2of copper was calculated according to the method proposed by Kivelson.(1)The results of these calculations are classified, together with the data quoted from the literature,1-5into four groups according to the kinds of ligand atoms, i.e., α2is 0.72 to 0.87 for 04- type complexes, 0.69 to 0.86 in N2O2, 0.74 to 0.82 in N4, and 0.54 to 0.62 in S4and S23N2type complexes, where 0, H and S refer the oxygen, nitrogen and sulfur atoms of ligands, respectively, that directly coordinate with copper.