Abstract:

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.

Key words: Boron halides, Lewis acidity, Principle of hard and soft acids and bases