Lewis Acidity of Boron Trihalides

The Lewis acidity of BF3, BF2Cl, BFCl2, and BCl3 in acid−base orbital interactions has been studied. We have derived the unoccupied reactive orbitals that show the maximum localization on the boron pπ atomic orbital overlapping with the lone-pair orbital of an electron donor and have evaluated the electrophilicity of the boron center in these compounds. The Lewis acidity of boron is shown to be controlled by two factors: localizability of the unoccupied reactive orbital on the boron pπ atomic orbital and the polarizability of the boron center. The former has been shown to be similar in magnitude in these boron halide compounds. Contrary to common belief, the conjugation between the boron atom and the attached halogen atoms is not necessarily stronger in BF3 relative to others. The trend observed in experiments and in theoretical calculations for BF3, BF2Cl, BFCl2, and BCl3 is interpreted in terms of these factors.