Is the Cation/π Interaction in Alkaline-Earth-Metal Dication/Benzene Complexes a Covalent Interaction?

The interaction energies of the alkaline-earth-metal dication/benzene complexes (dication = Mg2+ and Ca2+) were calculated at the MP2 (the second-order Møller−Plesset calculation) level using the 6-311G** basis set. The electrostatic and induction energies were calculated with the distributed multipoles and distributed polarizabilities model. Induction is mainly responsible for the very strong attractive interaction in these complexes. Electrostatic energies are considerably smaller than the induction energies. The complexes have significant attraction, even if the cations are well separated from the benzene, which indicates that short-range (covalent) interactions are not the major source of the attraction and that long-range interactions (induction and electrostatic interactions) are mainly responsible for the attraction. The calculations of benzene under an electric field show that the electric field produced by the dication is a cause of the C−C bond elongation in the alkaline-earth-metal dication/benzene complexes.