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

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 aluminum halides can be explained by the WolfsbergHelmholz (W-H) formula for resonance integral . The MO theory based predictions are valid only when the orbital interactions are strong enough. In the 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 aluminum halides can vary depending on the strength of 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.