Why edge inversion? Theoretical characterization of the bonding in the transition states for inversion in

As first noted by Dixon et al. (J Am Chem Soc 108:2461–2462, 1986), heavily fluorinated pyramidal phosphorus compounds, e.g., F

n

PH

(3-n)

with n>1, invert through a T-shaped transition state (edge inversion) rather than the D

3h

-like transition states (vertex inversion) found in the corresponding nitrogen compounds and less fluorinated phosphorus compounds. Subsequent studies by Dixon and coworkers established that this is a general phenomenon and has important chemical consequences. But what is the reason for the change in the structure of the transition state? Recent theoretical investigations have resulted in the discovery of a new type of chemical bond, the recoupled pair bond. In particular, it was found that recoupled pair bond dyads account for the hypervalency of the elements beyond the first row. In this paper, we show that recoupled pair bond dyads also account for the existence of the edge inversion pathway in heavily fluorinated phosphorus compounds and likely account for the presence of the lower energy inversion pathways in pyramidal compounds of other elements beyond the first row.