关于卤化硼的Lewis酸性强度的讨论

众所周知,卤化硼与Lewis强碱相互作用所呈现出的酸性强度顺序为BF3＜ BCl3＜ BBr3.这个顺序和人们所熟知的电负性规律矛盾,因为按照电负性变化规律,似乎BF3的酸性应该最强.为此化学工作者进行了解释,一个流行的说法是因为F含有孤对电子的2p轨道和B的空2p轨道能形成最有效的64键.但是后来的研究表明,Cl含有孤对电子的3p轨道和B的空2p轨道的重叠程度比相应的F含有孤对电子的2p轨道和B的空2p轨道的重叠程度大.为此,化学工作者提出了诸如电荷容量、价电子缺失数、前线轨道理论、锥形化能、最大硬度原理、最小亲电性原理等概念以解释卤化硼的酸性规律,本文对这些解释一一做了评点.

Discussion about Lewis Acidity of Boron Halides

It is well known that the acidity of boron halides increases in the order of BF₃ < BCl₃ < BBr₃ when interacting with strong Lewis bases. This is contradictory to what is expected from electronegativity, which suggests that boron trifluoride (BF₃) should be the strongest Lewis acid in the series. It is usually argued that the charge donation from fluorine 2p lone pairs into the empty boron 2p atomic orbital is more efficient and leads to the strongest π₄⁶ bond. But some theoretical results have shown that the overlap between the boron 2p orbital and the chlorine 3p orbital is larger than that between the boron 2p orbital and the fluorine 2p orbital. So many concepts, such as charge capacity, valence deficiency, frontier orbital theory, pyramidalization energy, maximum hardness principle, minimum electrophilicity principle etc., have been put forward to explain the acidity of boron halides. This article reviews these explanations.