Stability of borane - Adduct complexes: A G-2 molecular orbital study

Complexation energies of H3BXHn, and [H3BXHn-1](-) complexes (X = N, O, F, P, S, and Cl) (n = 3, 2, 1) have been computed at the G-2 level of theory. The formation of H3BXH3 (X = N, P) is found to be more favored than the formations of H3BXH2 (X = O, S) and H3BXH (X = F, Cl). The qualitative features of the molecular orbital interaction (the correlation diagrams) of H3BNH3 (C-3v symmetry group), H3BOH2 (C-s symmetry group), and H3BFH (Cs symmetry group) complexes are presented. These diagrams show that the sigma character of the B-X bond decreases and the sc character increases when the electronegativity of X increases and indicate that the B-X bond cannot be treated only in terms of the simplest model of the HOMOLUMO interaction (i.e., a two-level and two-electron model system). Two linear correlations were established and discussed. The first one was between proton affinities of the Lewis bases L (L = XHn and [XHn-1](-), n = 3, 2, 1) and complexation energies of the H3BL compounds calculated at the G-2 level of theory. The second correlation was between the B-11 NMR coupling constant (1)J(B-H) and the complexation energies of H3BL (L = OH-, PH2-, SH-, Cl-, NH3, and PH3).