Department of Chemistry, Regional Engineering College, Kurukshetra-132 119 Manuscript received 7 August 1987, revised 14 October 1987, accepted 73 November 2987 Copper(II) complexes of quadridentate ligands, viz., ethylenedibiguanide, trimetby­lenedibiguanide, m-phenylenedibiguanide, piperazinedibiguanide, and bidentate ligand phenylbiguanide have been studied by epr spectroscopy in polycrystalline form and in the corresponding NiII complex matrix. Epr spectra of the complexes hi the corres­ponding NiII complex matrix exhibit nine nitrogen superhyperfiue lines on the high field 3/2 ↔ 3/2 copper hyperfine splitting components indicating the presence of four equivalent nitrogen atoms around the CuII ion. The evaluation of the covalency para­meters from the epr data indicates that the unpaired electron of CuII spends about 31 —36% of its time in the nitrogen donor sites of the ligands. The epr spectra of the complexes suggest the presence of CuN4 coordination sphere in the complexes. A comparison of the bonding parameters of the Cull dibiguanide complexes with those of the Curl complexes of biguanide and phenylbiguanide indicates that the bridging group in quadridentate dibiguanides has negligible effect on the epr spectra and bonding para­meters. A comparison of the bonding parameters of the present complexes with those of the CuII complexes of tetrapenylporphine and phthalocyanine indicates that biguanides and dibiguanides rank among strong field ligands.