The π-Electron-Accepting Ability of the Boron Atom in Ethynylboranes and Related Compounds – An Approximate Weight Computation for Resonance Structures⋆

Abstract

A theoretical analysis was performed to quantify the π-electron-accepting ability of the boron atom in ethynylboranes. An expansion technique was employed which permits to obtain a set of localized bonding schemes and their weights from a delocalized molecular orbital determinantal wavefunction. The derived manifold of bonding schemes is close to the classical resonance hybrid used in organic chemistry (valence-bond description). We quantified the π-electron transfer into the empty π-orbital of the boron atom by investigating nine model compounds where substituents with π-electron-donating ability are adjacent to a boron atom. This led to an ordering of the substituents according to their electron-donating ability towards boron. The boron atom hesitates to accept π-electrons from the ethynyl group in ethynylboranes in particular when good π-donors like amino groups are present. The π-electron donation from the vinyl group to the adjacent boron centre is slightly stronger than from the ethynyl group. Nitrogen lone-pair electrons are easily transferred to a neighbouring boron centre. Bonding schemes and their weights are in line with computed bond lengths and rotational barriers. Moreover, our theoretical results rationalize previous NMR and X-ray experiments and are in line with the reactivity of related compounds. It is demonstrated that bond lengths alone do not necessarily correlate with the degree of π-bonding and should be discussed with caution. The analysis is substantiated by showing that weights for covalent bonding schemes, as obtained from the simple restricted closed-shell MO determinant, correlate with bond strengths. Furthermore, a correlation of bonding-scheme weights with quantities based on the fragment orbital approach is presented. This novel correlation elucidates molecular properties which determine the extent of the π-electron transfer to the boron atom and permits a quantitative interpretation and prediction of intramolecular π-bonding.