Abstract Six new mixed copper(II) chelates with N,N,N′,N′-tetramethylethylenediamine and a bidentate ligand, i.e., [Cu tmen en] (ClO4)2, [Cu tmen en] (NO3)2·H2O, [Cu tmen en](SO4)·4H2O, [Cu tmen gly](ClO4), [Cu-tmen ox] ·4H2O and [Cu tmen aca](ClO4) were prepared, and their visible absorption spectra were compared with those of the bis-chelate ions and molecules, [Cu en2]2+, [Cu gly2], [Cu ox2]2−, and [Cu aca2]. The results obtained indicate that steric factors play an important role in the formation and structure of these mixed chelates. The effects of various organic solvents on the electronic spectra of these chelates were also studied. (tmen=N,N,N′,N′-tetramethylethylenediamine, en=ethylenediamine, gly=glycinate ion, ox=oxalate ion, aca=acetylacetonate ion.)