Hypervalency and recoupled pair bonding in the p-block elements

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 s² and p² 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.