The complex fluxional interconversions between otherwise very similar phosphonium bromides and chlorides R PX X (R=Alk, Ar, X=Cl or Br) were studied by NMR techniques. Their energy barriers are typically ca. 11 kcal mol , but rise rapidly as bulky groups are attached to phosphorus, revealing the importance of steric factors. In contrast, electronic effects, as measured by Hammett analysis, are modest (ρ 1.46) but still clearly indicate negative charge flow towards phosphorus in the transition state. Most significantly, detailed analysis of the exchange pathways unequivocally, and for the first time in any such process, shows that nucleophilic attack of the nucleophilic anion on the tetrahedral centre results in inversion of configuration.

The complex fluxional interconversions between otherwise very similar phosphonium bromides and chlorides R 3 PX + X À (R = Alk, Ar,X= Cl or Br) were studied by NMR techniques.T heir energy barriers are typically ca. 11 kcal mol À1 ,b ut rise rapidly as bulky groups are attached to phosphorus,r evealing the importance of steric factors.I n contrast, electronic effects,a sm easured by Hammett analysis, are modest (1 1.46) but still clearly indicate negative charge flowt owards phosphorus in the transition state.M ost significantly,d etailed analysis of the exchange pathways unequivocally,a nd for the first time in any such process,s hows that nucleophilic attack of the nucleophilic anion on the tetrahedral centre results in inversion of configuration.

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Unlike an unruly brolly or the striking lily of the Nile, the celebrated umbrella‐like SN2 inversion has not been directly observed until now. In their Communication on page 1480 ff., K. Nikitin, D. G. Gilheany, et al. demonstrate direct observation of inversion in a low‐temperature NMR‐EXSY experiment using an inseparable mixture of isomers. Fast nucleophilic attack of a halide ion on a phosphonium center has all the characteristics of an associative process and leads to inversion of the P configuration.