Just a short update here. In Chapter 5.1.2 we discuss nucleophilic substitution at heteroatoms. Unlike the paradigmatic case for substitution at carbon, which proceeds via the S_N2 mechanism. Nucleophilic substitution at second-row atoms (S, Si, P) appears to follow an addition-elimination pathway. Bickelhaupt¹ now adds a more thorough computational examination of nucleophilic substitution at phosphorus. He looked at a few identity reactions involving tricoordinate P, namely

X^– + PH₂X → PH₂X + X^–

X^– + PF₂X → PF₂X + X^–

X^– + PCl₂X → PCl₂X + X^–

where X is chloride or hydroxide. In all cases the only critical point located on the potential energy surface is for a tetracoordinate intermediate. Shown in Figure 1 are the intermediates for the reaction OH^– + PH₂OH and Cl^– + PCl₃. This result is consistent with the studies of nucleophilic substitution at sulfur and silicon.

(a) xyz file

(b) xyz file

Figure 1. OLYP/TZ2P optimized intermediate for the reaction (a) OH^– + PH₂OH
and (b) Cl^– + PCl₃.

References:

(1) vanBochove, M. A.; Swart, M.; Bickelhaupt, F. M., "Nucleophilic Substitution at Phosphorus (S_N2@P): Disappearance and Reappearance of Reaction Barriers," J. Am. Chem. Soc. 2006, 128, 10738-10744, DOI: 10.1021/ja0606529