Bonding studies of compounds of boron and the Group IV elements. Part II. Ionisation potentials of boron halides and mixed halides by electron impact and by molecular orbital calculations

Abstract

All the twenty possible ions of the type BX₃⁺, BX₂Y⁺, and BXYZ⁺(X, Y, and Z = F, Cl, Br, or I) have been identified mass spectrometrically. The mixed boron halides were obtained from the simple halides by redistribution reactions in the spectrometer source. The appearance potentials of these ions were calculated from the observed ionisation efficiency curves, using the semi-logarithmic-plot method. The values for the first ionisation potentials for BX₃ agree closely with the most recent data from photoelectron spectroscopy.

Various semi-empirical MO theories were applied to calculate the first ionisation potentials for the molecules BX₃, BX₂Y, and BXYZ. Close agreement with the experimental data was found using an extended Hückel approach, modified to account for the considerable charge-drift occurring in these compounds, which was then employed to calculate the electron distribution. The results indicate, that, contrary to the usual assumptions, π back-donation decreases in the series BI > BBr ≈ BCl BF; the σ-charge drift is the dominant factor in deciding overall bond polarity.