A set of [Cu (I) L n (C 2 H 4)] q (q =-1, 0, or 1) complexes modelling systems of experimental interest were 5 studied by DFT calculations to analyze the Cu (I)-ethylene bonding using NBO and CDA analyses. All complexes are better viewed as donor-acceptor complexes between a d 10 Cu (I) center and ethylene. Back-donation depends significantly on the nature and number of the ancillary ligands, hence on the coordination sphere at copper. Back-donation is shown to vary more with the nature of the ligands than donation and to increase significantly with the number of ancillary ligands. However, even with strongly 10 donating ligands such as alkyl (modelled by CH 3), there is no tendency of forming a metallacyclopropane. This can lead to reconsider the mechanisms of alkylation of olefin catalyzed by copper complexes.