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 Mg²⁺, the calculated Cu ²⁺ binding energies are about 1.5 and 2 times larger than Mg ²⁺ 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 ^+• or thiourea^+• radical cations and Cu ⁺.