Mixed-ligand Cu2+ ternary complexes, formed by an aromatic diimine and a second ligand with O donor atoms, show a higher than expected stability. To understand the factors affecting the stability of these systems, we performed a density functional study of [Cu(H2O)(5)](2+), [Cu(N-N)(H2O)(3)](2+), and [Cu(N-N)(O-O)H2O] {N-N is 1, 10-phenanthroline, 5-nitro-1, 10-phenanthroline, or 3,4,7,8-tetramethyl-1,10-phenanthroline; and O-O is oxalate}. In the present study, full geometry optimization (B3LYP/3-21G**) has been performed without symmetry constraints and a comparison with some available experimental results has been made. Bond distances, equilibrium geometries, harmonic frequencies, and net atomic charges from Mulliken populations are presented. Since the principle of hard and soft acids and bases has been 'widely used to explain the stability' of these complexes, we also calculated and analyzed the global hardness and the local softness. The results of the global hardness do not support the commonly held idea that harder acids will preferably bind to harder ligands, while softer acids will bind lo softer ligands. interestingly, local softness and electron affinity correlate well with the formation constants of these compounds and provide an explanation of the reactivity behavior. The present results may help to rationalize the stability and reactivity of these systems.