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The π‐Electron‐Accepting Ability of the Boron Atom in Ethynylboranes and Related Compounds – An Approximate Weight Computation for Resonance Structures ☆
Author(s) -
Bachler Vinzenz,
MetzlerNolte Nils
Publication year - 1998
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/(sici)1099-0682(199806)1998:6<733::aid-ejic733>3.0.co;2-5
Subject(s) - chemistry , boron , delocalized electron , atom (system on chip) , computational chemistry , valence electron , covalent bond , electron deficiency , crystallography , lone pair , electron , electron pair , resonance (particle physics) , atomic physics , molecule , organic chemistry , quantum mechanics , physics , computer science , embedded system
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.