A high-level ab initio investigation of identity and nonidentity gas-phase S_N2 reactions of halide ions with halophosphines

The high-level ab initio procedures G2(+) and G2(+)[ECP(S)]have been employed in an investigation of S_N2 reactions at neutral tri-coordinated phosphorus. The process has been modeled by identity and nonidentity substitution reactions involving a series of halophosphines PH₂-X and halide ions. We find that the reaction proceeds without an intervening barrier by way of a tetra-coordinated phosphorus anion intermediate (X-PH₂-Y^-). This contrasts with the corresponding process for the carbon and nitrogen analogues, where the tetra-coordinated species is a transition structure. The threshold for inversion of the phosphorus intermediate is found to lie below the reaction energy in the cases where F^- is the leaving group, but above it in all the other cases. The S_N2 reaction will therefore lead to racemization when F^- is expelled. In the other cases, it is possible in principle that the reaction can be controlled to proceed with inversion, but because of the relatively low barriers for inversion, this is not a very likely outcome. We predict that the tetra-coordinated intermediate should be detectable, if not isolable, and that the S_N2 reaction at neutral tri-coordinated phosphorus is exothermic when the reactant halide ion is more electronegative than the product halide ion, and endothermic when the reverse applies.