THE PI-ELECTRON-ACCEPTING ABILITY OF THE BORON ATOM IN ETHYNYLBORANESAND RELATED-COMPOUNDS - AN APPROXIMATE WEIGHT COMPUTATION FOR RESONANCE STRUCTURES

A theoretical analysis was performed to quantify the pi-electron-accep ting ability of the boron atom in ethynylboranes. An expansion techniq ue was employed which permits to obtain a set of localized bonding sch emes and their weights from a delocalized molecular orbital determinan tal wavefunction. The derived manifold of bonding schemes is close to the classical resonance hybrid used in organic chemistry (valence-bond description). We quantified the pi-electron transfer into the empty p i-orbital of the boron atom by investigating nine model compounds wher e substituents with pi-electron-donating ability are adjacent to a bor on atom. This led to an ordering of the substituents according to thei r electron-donating ability towards boron, The boron atom hesitates to accept pi-electrons from the ethynyl group in ethynylboranes in parti cular when good pi-donors Like amino groups are present. The pi-electr on donation from the vinyl group to the adjacent boron centre is sligh tly stronger than from the ethynyl group. Nitrogen lone-pair electrons are easily transferred to a neighbouring boron centre. Bonding scheme s and their weights are in line with computed bond lengths and rotatio nal barriers. Moreover, our theoretical results rationalize previous N MR and X-ray experiments and are in Line with the reactivity of relate d compounds. It is demonstrated that bond lengths alone do not necessa rily correlate with the degree of pi-bonding and should be discussed w ith caution. The analysis is substantiated by showing that weights for covalent bonding schemes, as obtained from the simple restricted clos ed-shell MO determinant, correlate with bond strengths. Furthermore, a correlation of bonding-scheme weights with quantities based on the fr agment orbital approach is presented. This novel correlation elucidate s molecular properties which determine the extent of the pi-electron t ransfer to the boron atom and permits a quantitative interpretation an d prediction of intramolecular pi-bonding.