Hydride transfer from the silicate anion then leads to phosphorane M5, which can isomerise to form M6. This is very different from the mechanistic proposal of Dobrovetsky for HDF by geometrically constrained  3 -P systems, where it was suggested that PhSiH3 reacts directly with the fluorophosphorane through a transition state that involves concerted hydride transfer to P and fluoride transfer to Si. The experiments described above showed that for simple trialkylphosphines, phosphonium ions are intermediates and these do not react directly with the neutral silanes. It may be that the positive charge on Dobrovetsky's constrained  3 -P systems disfavours this pathway and leads to this divergence in mechanistic behaviour.