Mechanism of the [RhF(PPh₃)₃] to cis-[RhPh(PPh ₃)₂(PFPh₂)] interconversion: P-C activation and F/Ph exchange via a metallophosphorane pathway

Density functional calculations have been carried out to model various pathways for F/Ph exchange in [RhF(PPh₃)₃]. Calculations on the full experimental system showed this process to occur via a novel pathway involving initial attack of fluoride on phosphine. This results in the formation of a metallophosphorane intermediate featuring a 5-coordinate phosphorus center. From this species P-Ph activation is very facile and leads to the F/Ph-exchanged product, cis-[RhPh(PPh₃)₂(PFPh ₂)]. The computed activation energy of 22.3 kcal/mol is in excellent agreement with the experimental value. Alternative pathways based on initial Ph group transfer are significantly higher in energy. A key factor favoring the metallophosphorane route is the reduction in coordination number at the metal center that occurs in the initial fluoride transfer step; in contrast Ph group transfer creates a more sterically encumbered metal center. The metallophosphorane pathway indicates that "hard" donor species such as fluoride can induce facile P-C bond activation. This novel mechanism of phosphine ligand disruption may have general significance for the stability of transition metal-phosphine complexes in the presence of such species.