The importance of the oxidative character of doubly charged metal cations in binding neutral bases. [Urea-M](2+) and [thiourea-M](2+) (M = Mg, Ca, Cu) complexes

Different analysis of the electron density, namely AIM, ELF and NBO, yield coherent descriptions of the bonding in [urea-M](2+) and [thiourea-M](2+) (M = Mg, Ca, Cu) complexes, whose geometries have been optimized at the B3LYP/6-311+G(d, p) level of theory. The interactions of these two bases with the alkaline-earth dications are essentially electrostatic, whereas the bonding with Cu2+ has a non-negligible covalent character. As a consequence and in spite of the fact that Cu2+ has practically the same ionic radius as Mg 2+, the calculated Cu2+ binding energies are about 1.5 and 2 times larger than Mg2+ and Ca2+ binding energies, respectively. The main difference, however, between Mg2+ or Ca2+ with respect to Cu2+ complexes is due to the fact that Cu2+ is able to oxidize the base. Hence, both [urea-Cu](2+) and [thiourea-Cu](2+) complexes can be viewed as the interaction between urea(+center dot) or thiourea(+center dot) radical cations and Cu+.