The final step involves RE of 2 from M6 via TS61 (Figure ). This transition state is more concerted than TS12, although still somewhat asynchronous, presumably because direct concerted RE from the axial and equatorial positions of a phosphorane is symmetry forbidden. 75 TS61 is again Meisenheimerlike, although less so than in the constrained  3 -P systems of Radosevich, where the C-H bond length of a related RE TS is 1.33 Å, and Dobrovetsky where it is 1.53 Å. It seems as though the substituents and geometric environment around phosphorus have a significant impact on the RE process. These final steps give rise to the energetic span for the reaction, which is defined by the turnoverdetermining intermediate (TDI) M5 and transition state (TDTS) TS61, and give an overall barrier for the catalytic reaction of 81 kJ mol -1 . This low barrier is consistent with the observed fast reaction between 1 and Ph2SiH2 when a sterically relatively small phosphine like P n Bu3 is used as the catalyst (full conversion in 20 minutes at 20 o C in MeCN). The calculated energetic span for HDF of 1 by a constrained  3 -P system was significantly larger (140 kJ mol -1 ), which is consistent with the slower reactions observed in that study.