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 Cu(2+) has a non-negligible covalent character. As a consequence and in spite of the fact that Cu(2+) has practically the same ionic radius as Mg(2+), the calculated Cu(2+) binding energies are about 1.5 and 2 times larger than Mg(2+) and Ca(2+) binding energies, respectively. The main difference, however, between Mg(2+) or Ca(2+) with respect to Cu(2+) complexes is due to the fact that Cu(2+) is able to oxidize the base. Hence, both [urea-Cu](2+) and [thiourea-Cu](2+) complexes can be viewed as the interaction between urea(+*) or thiourea(+*) radical cations and Cu(+).