Hypervalency and recoupled pair bonding in the p-block elements

Abstract

The nature of the bonding in hypervalent molecules has long been a topic of discussion with a number of explanations being offered to rationalize the apparent violation of the octet rule. By examining the formation of the second row fluorides, XF n −1 + F → XF n (X = P, S, Cl), we found that a new type of bond is present in these compounds, the recoupled pair bond, which is distinct from other types of chemical bonds. A recoupled pair bond occurs when it is energetically favorable to uncouple an existing pair of electrons to form bonds, which is favorable for most of the valence s 2 and p 2 pairs in P, S, and Cl. The interplay between the formation of normal covalent bonds and recoupled pair bonds in the XF n species readily explains the structures of these species as well as the large variations observed in the XF n −1–F bond energies. In addition, the formation of recoupled pair bonds leads to the presence of unexpected low-lying excited states in the XF n molecules. We also consider the significant parallels that exist between the XF n families and the YH n families, where Y = Be, B and C. The latter species also involve recoupled pair bonding, which accounts for the divalence of Be, the trivalence of B and the tetravalence of C without invoking the concept of hybridization.