Lewis Acidity Trend of Boron and Aluminium Trihalides: If Not π Back‐Bonding, What Else?

• Published in: Chemphyschem Vol. 24; no. 5; pp. e202200761 - n/a
• Main Authors: Thayyil, Ashith, Parambil, Priyakumari Chakkingal
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 01.03.2023
• Subjects: Aluminum; Bonding strength; Boron; Boron and Aluminium trihalides; Chemical bonds; Electronegativity; GKS-EDA; Halides; Lewis acidity; Lewis base; Molecular orbitals; Orbital interaction; π back-bonding; Lewis acidity; GKS-EDA; Boron and Aluminium trihalides; π back-bonding; Orbital interaction
• ISSN: 1439-4235; 1439-7641; 1439-7641
• DOI: 10.1002/cphc.202200761

Lewis acidity trend of boron trihalides is a subject that has received a variety of explanations, and still, the simple π back‐bonding based one is believed by most, perhaps because of its simplicity, irrespective of opposing findings. Herein we try to give an alternative explanation based on qualitative Molecular Orbital (MO) theory and support that quantitatively by Generalized Kohn‐Sham Energy Decomposition Analysis. While the role of orbital overlap on the orbital interaction energy is widely known, the role of electronegativity of the atoms involved is often overlooked. Here we find that the Lewis acidity trend of boron and aluminium halides can be explained by the Wolfsberg‐Helmholz (W−H) formula for resonance integral. The MO theory‐based predictions are valid only when the orbital interactions are strong enough. In weakly interacting systems, the effect of orbital interactions can be overshadowed by other effects such as Pauli repulsion, dispersion, etc. Thus the Lewis acidity trend of boron and aluminium halides can vary depending on the strength of the interacting Lewis base. We believe that this work would enable one to gain a better understanding not only on the Lewis acidity of boron trihalides and its heavy analogs but also on a variety of related problems such as the stronger π acidity of CS compared to CO and weaker π bonding between heavy atoms. The Lewis acidity trend of boron and aluminium halides is explained based on qualitative MO theory and supported quantitatively by Generalized Kohn‐Sham Energy Decomposition Analysis. The strength of a Lewis acid is not absolute, but depends on the donor strength of the interacting Lewis Base (LB). MO theory can successfully predict the trends in Lewis acidity, only if the orbital interaction energy is large enough.