Ligand Close-Packing and the Lewis Acidity of BF(3) and BCl(3)

The unexpected greater Lewis acidity of BCl(3) than BF(3) with respect to strong bases such as NH(3) has been the subject of much discussion. A number of explanations have been proposed, among which the most popular and most widely quoted is that stronger back-donation from fluorine than from chlorine decreases the availability of the otherwise empty 2p orbital on boron from accepting an electron pair from a base. In contrast, toward weak bases such as CO, BF(3) is a stronger Lewis acid than BCl(3). We have reinvestigated the relative acid strengths of BF(3) and BCl(3) toward Lewis bases by calculating geometries and atomic charges for the following adducts: BF(3).NH(3), BF(3).N(CH(3))(3), BF(3).OH(2), BF(3).O(CH(3))(2), BCl(3).NH(3), BCl(3).N(CH(3))(3), BCl(3).OH(2), and BCl(3).O(CH(3))(2). Our results show that the halogen ligands remain close-packed throughout the formation of an adduct and that the bond lengths increase accordingly. It takes more energy to lengthen the short strong BF bonds than the longer weaker BCl bonds and it is for this reason that BCl(3) is a stronger Lewis acid than BF(3) toward a strong base such as NH(3). In contrast, in the formation of a complex with a weak base such as CO, the BX(3) is barely distorted from planarity and so the acidity of BF(3) is greater than that of BCl(3) because the charge on boron is greater in BF(3) than BCl(3).