Although copper based complexes have been widely used in homogeneous catalysis, more recently they are attracting considerable attention as pharmaceutical therapeutic agents. Of paramount importance in their efficacy of use is their structure and electronic properties, which can be thoroughly probed using advanced EPR techniques. In this study, a series of [Cu(acac)(N-N)](+) Casiopeina type complexes were investigated, bearing a series of diimine N-N ligands (including bipy, phen, Py-bipy and dppz). All complexes displayed rhombic g and (Cu)A tensors, although the extent of rhombicity was dependent on the N-N ligand. Greater Cu(II)-N2 in-plane distortion, away from the square planar arrangement, was detected by CW W-band EPR for the smaller bipy and phen ligands compared to the larger Py-bipy and dppz ligands. Changes in ligand spin density distributions (over the H-1 and N-14 nuclei) were revealed by CW Q-band ENDOR. The largest components of the H-1 imine and N-14 hyperfine coupling decreased as the ligand size increased, following the trend bipy > phen > Py-bipy > dppz. These results indicate how even small structural and electronic (spin density) perturbations within the Casiopeina family of Cu(II) complexes can be probed by advanced EPR methods. (c) 2020 The Author(s). Published by Elsevier Inc. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).