A density functional study of the reactivity and stability of mixed coppercomplexes. Is hardness the reason?

Mixed-ligand Cu2+ ternary complexes, formed by an aromatic diimine and a se cond ligand with O donor atoms, show a higher than expected stability. To u nderstand the factors affecting the stability of these systems, we performe d a density functional study of [Cu(H2O)(5)](2+), [Cu(N-N)(H2O)(3)](2+), an d [Cu(N-N)(O-O)H2O] {N-N is 1, 10-phenanthroline, 5-nitro-1, 10-phenanthrol ine, or 3,4,7,8-tetramethyl-1,10-phenanthroline; and O-O is oxalate}. In th e present study, full geometry optimization (B3LYP/3-21G**) has been perfor med without symmetry constraints and a comparison with some available exper imental results has been made. Bond distances, equilibrium geometries, harm onic frequencies, and net atomic charges from Mulliken populations are pres ented. Since the principle of hard and soft acids and bases has been 'widel y used to explain the stability' of these complexes, we also calculated and analyzed the global hardness and the local softness. The results of the gl obal hardness do not support the commonly held idea that harder acids will preferably bind to harder ligands, while softer acids will bind lo softer l igands. 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.