A high-level ab initio investigation of identity and nonidentity gas-phase SN2 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(N)2 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(N)2 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(N)2 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. (C) 2001 Elsevier Science B.V.