First-Row Anomaly

The dramatic differences between the properties of molecules formed from the late p-block elements of the first row of the periodic table (N–F) and those of the corresponding elements in subsequent rows is well recognized as the first row anomaly. Certain properties of the atoms, such as the relative energies and spatial extents of the ns and np orbitals, can explain some of these differences, but not others.

In this Account, we summarize the results of our recent computational studies of the halides of the late p-block elements. Our studies point to a single underlying cause for many of these differences: the ability of the late p-block elements in the second and subsequent rows of the periodic table to form recoupled pair bonds and recoupled pair bond dyads with very electronegative ligands. Recoupled pair bonds form when an electron in a singly occupied ligand orbital recouples the pair of electrons in a doubly occupied lone pair orbital on the central atom, leading to a central atom-ligand bond. Recoupled pair bond dyads occur when a second ligand forms a bond with the orbital left over from the initial recoupled pair bond.

Recoupled pair bonds and recoupled pair bond dyads enable the late p-block elements to form remarkably stable hypervalent compounds such as PF₅ and SF₆ and lead to unexpected excited states in smaller halides of the late p-block elements such as SF and SF₂. Recoupled pair bonding also causes the F_n-1X–F bond energies to oscillate dramatically once the normal valences of the central atoms have been satisfied. In addition, recoupled pair bonding provides a lower-energy pathway for inversion in heavily fluorinated compounds (PF₃ and PF₂H, but not PH₂F and PH₃ ) and leads to unusual intermediates and products in reactions involving halogens and late p-block element compounds, such as (CH₃)₂S + F₂.