Nature and Strength of Lewis Acid/Base Interaction in Boron and Nitrogen Trihalides.

We have quantum chemically investigated the bonding between archetypical Lewis acids and bases. Our state-of-the-art computations on the X B-NY Lewis pairs have revealed the origin behind the systematic increase in B-N bond strength as X and Y are varied from F to Cl, Br, I, H. For H B-NY , the bonding trend is driven by the commonly accepted mechanism of donor-acceptor [HOMO(base)-LUMO(acid)] interaction. Interestingly, for X B-NH , the bonding mechanism is determined by the energy required to deform the BX to the pyramidal geometry it adopts in the adduct. Thus, Lewis acids that can more easily pyramidalize form stronger bonds with Lewis bases. The decrease in the strain energy of pyramidalization on going from BF to BI is directly caused by the weakening of the B-X bond strength, which stems primarily from the bonding in the plane of the molecule (σ-like) and not in the π system, at variance with the currently accepted mechanism.