Large Effect on Borane Bond Dissociation Energies Resulting from Coordination by Lewis Bases

Abstract

Ab initio molecular orbital calculations at the G-2 and CBS-4 levels of theory were used to determine homolytic bond dissociation energies (BDE's) for the B−H bonds in a series of donor−acceptor complexes of borane. The B−H bonds of four-coordinate boron were found to be weaker than those of three-coordinate boron. The effect of complexation on BDE varied from very small, in the cases of NH3 and H2O, to as much as 30−50 kcal/mol, in the cases of formaldehyde, CO, and HCN. The BDE's were not found to correlate with the strength of coordination. However, they were closely correlated with the degree to which spin density in the radical was delocalized away from boron and onto the associated Lewis base. The presence of either a π system or, to some extent, a second-row element such as phosphorus or sulfur promoted such delocalization. Delocalization of spin density onto carbon appeared to be particularly favorable, and to correlate with particularly low B−H BDE's. The BDE's in 4-coordinate complexes of borane...