Nucleophilic substitution at phosphorus (S_N2@P): Disappearance and reappearance of reaction barriers

Pentacoordinate phosphorus species play a key role in organic and biological processes. Yet, their nature is still not fully understood, in particular, whether they are stable, intermediate transition complexes (TC) or labile transition states (TS). Through systematic, theoretical analyses of elementary S_N2@C, S_N2@Si, and S_N2@P reactions, we show how increasing the coordination number of the central atom as well as the substituents' steric demand shifts the S_N2@P mechanism stepwise from a single-well potential (with a stable central TC) that is common for substitution at third-period atoms, via a triple-well potential (featuring a pre- and post-TS before and after the central TC), back to the double-well potential (in which pre- and postbarrier merge into one central TS) that is well-known for substitution reactions at carbon. Our results highlight the steric nature of the S_N2 barrier, but they also show how electronic effects modulate the barrier height.