Ab initio molecular orbital study of the substituent effect on ammonia and phosphine–borane complexes

Abstract

The complexation energies of H 3BXH 3− n F n ( X=N, P; n=0–3) and the proton affinities of XH 3– n F n compounds have been investigated at the G2(MP2) level of theory. The G2(MP2) results show that the phosphine complexes are more stable than the corresponding ammonia ones. Increasing fluorine substitution on nitrogen atom reduces both the basicity of NH 3− n F n and the stability of ammonia complexes. For the phosphine complexes, the successive fluorine substitution on the phosphine increase the stability of H 3BPH 3− n F n complexes although the reduction of the basicity of the PH 3– n F n ligands with this substitution. The NBO partitioning scheme shows that the stability of the phosphine complexes was related to the hypeconjugation effect. It also proves that the shortening of the P–H and X–F bond lengths, upon complexation, is due to an increasing ‘s’ character in these bonds.