On the Stability of Interactions between Pairs of Anions - Complexes of MCl3 - (M=Be, Mg, Ca, Sr, Ba) with Pyridine and CN.

The ability of the central M atom of the MCl3 - anion, with M=Be, Mg, Ca, Sr, Ba, to engage in a noncovalent bond with an approaching nucleophile is gauged by ab initio methods. The N atom of pyridine forms a M⋅⋅⋅N bond with an interaction energy between 12 and 21 kcal mol-1 , even though the π-hole above the M atom is not necessarily positive in sign. Despite a strong Coulombic repulsion between two anions, CN- is also able to approach the M atom so as to engage in a metastable complex that is higher in energy than the individual anions. The energy barrier separating this complex from its constituent anion pair is roughly 20 kcal mol-1 . Despite the endothermic formation reaction energy of the CN- ⋅⋅⋅MCl3 - complex, the electron topology signals a strong interaction, more so than in pyridine⋅⋅⋅MCl3 - with its exothermic binding energy. The dianionic complex is held together largely on the strength of interorbital interactions, thereby overcoming a repulsive electrostatic component. The latter is partially alleviated by the pyramidalization of the MCl3 unit which makes its π-hole more positive. The complex sinks below the separate monomers in energy when the system is immersed in an aqueous medium, with a binding energy that varies from as much as 20 kcal mol-1 for Be down to 1.2 kcal mol-1 for Ba.