On the Stability of Interactions between Pairs of Anions – Complexes of MCl<sub>3</sub><sup>−</sup> (M=Be, Mg, Ca, Sr, Ba) with Pyridine and CN<sup>−</sup>
Wiktor Zierkiewicz, Rafał Wysokiński, Mariusz Michalczyk, Steve Scheiner
Abstract
Abstract The ability of the central M atom of the MCl 3 − 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 − ⋅⋅⋅MCl 3 − complex, the electron topology signals a strong interaction, more so than in pyridine⋅⋅⋅MCl 3 − 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 MCl 3 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.