Bonding in triphenylphosphine borane and related molecules

Abstract

The enthalpies of reaction of diborane and boron trifluoride with several tertiary alkyl and aryl phosphines in benzene solution have been measured by direct calorimetry. Correlation of the results with the sums of the Taft polar substitution constants of the bases indicates that a special small stabilization factor operates in the triarylphosphine boranes, but is absent in the trialkyl analogues and in triphenylphosphine boron trifluoride. The stabilization amounts to ∼3.8 ± 1.3 kcal mole^–1 and may be attributed to enhanced back-π-bonding between the borane group and a vacant d-orbital on the phosphorus. The fact that this enhanced stabilization does not occur in the triphenylphosphine-boron trifluoride adduct inducates that it is not due to conjugation of the aromatic π-electrons of the base with a suitable orbital on the phosphorus. Our results also show that : (i) the donor-acceptor bonds in the trialkylphosphine borane adducts are significantly stronger than those in the triarylphosphine boranes; (ii) the donor-acceptor bonds in the phosphine-boron trifluoride adducts are much weaker (∼20 kcal mole^–1) than those in the phosphine borane adducts.