Minimum electrophilicity principle in Lewis acid-base complexes of boron trihalides

The hardness and electrophilicity values for the complexes of some boron trihalides; BX3 (X = F, Cl and Br) with strong (NH3, H2O, N(CH3)3 and O(CH3)2) and weak (CO, CH3F, HCN and CH3CN) bases have been calculated using different levels of theory (HF/6-31G∗∗, HF/6-311++G∗∗, B3LYP/6-31G∗∗ and B3LYP/6-311++G∗∗). For both strong and weak bases the hardness values decrease from BF3-base to BBr3-base. This indicates more acidity strength of BF3 among the considered boron trihalides, which is not in agreement with experiment for the interaction of BX3 with strong bases. On the other hand, in the case of strong bases the obtained electrophilicities increase in the order BF3-base < BCl3-base < BBr3-base; whereas for weak bases this trend is reversed. Therefore, according to the Minimum Electrophilicity Principle; MEP, the strength of the Lewis acidity of BX3 against strong bases is expected to be in the order BF3 < BCl3 < BBr3; and the reverse trend is predicted for the weak bases. This is in full agreement with the experimental evidences. The extent of dihedral angle distortions of BX3 are also calculated in each series of BX3-strong base complexes and it is shown that there is significant correlation between the obtained electrophilicity values and the extents of distortion for each series.