A study of the donor-acceptor bond in phosphine-borane complexes

A nuclear magnetic resonance study of 15 phosphine-boranes has been undertaken with the view to investigating the electronic nature of the P-B dative bond. A correlation between the ³¹P-¹¹B coupling constant and the base strength of the phosphines toward BH₃ has been established on the basis of selected base displacement reactions. The observed sequence of phosphine base strengths and J [subscript PB] is inconsistent with the borane hyperconjugative model per se although this effect is not excluded. The results are best interpreted in terms of a combination of [sigma] effects and dative [pi]-bonding from the phosphine substituents. Similarly, the ¹¹B-¹H coupling constant data do not discriminate between the sigma-bonding and borane hyperconjugative models. Several ³¹P and ¹¹B chemical shifts have also been measured but these data do not provide definitive information concerning the nature of the P-B bond. The ³¹P coordination chemical shifts are interpreted in terms of changes in the paramagnetic and diamagnetic contributions. It is noted that the trends in the coordination chemical shifts for the complexation of phosphines with the borane unit is similar to that observed for the isoelectronic oxygen atom. The general trend of the ¹H chemical shifts of the borane moiety suggests that the amount of charge transferred to this unit is proportional to the basicity of the phosphine.