Thayyil Ashith, Parambil Priyakumari Chakkingal
Department of Chemistry, Indian Institute of Technology, Palakkad, 678 557, India.
Chemphyschem. 2023 Mar 1;24(5):e202200761. doi: 10.1002/cphc.202200761. Epub 2022 Dec 5.
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.
三卤化硼的路易斯酸性趋势是一个已得到多种解释的主题，然而，基于简单π反馈键的解释仍被大多数人所认可，或许是因其简单性，尽管存在相反的研究结果。在此，我们尝试基于定性分子轨道（MO）理论给出一种替代解释，并通过广义科恩-沙姆能量分解分析进行定量支持。虽然轨道重叠对轨道相互作用能的作用广为人知，但所涉及原子的电负性作用却常常被忽视。在此我们发现，硼和铝的卤化物的路易斯酸性趋势可用共振积分的沃尔夫斯堡-赫尔姆霍兹（W-H）公式来解释。基于MO理论的预测仅在轨道相互作用足够强时才有效。在弱相互作用体系中，轨道相互作用的影响可能会被诸如泡利排斥、色散等其他效应所掩盖。因此，硼和铝的卤化物的路易斯酸性趋势会因相互作用的路易斯碱的强度不同而有所变化。我们相信，这项工作不仅能使人们更好地理解三卤化硼及其重类似物的路易斯酸性，还能更好地理解各种相关问题，比如CS相较于CO更强的π酸性以及重原子之间较弱的π键。
