One final observation is important to note in a mechanistic context. As described above, stoichiometric reaction of PR3 with 1 in MeCN leads to rapid formation of the reductive coupling product 3 (Scheme 2, 3) alongside the formation of the difluorophosphorane PR3F2. Dobrovetsky and coworkers observed similar reactivity when a geometrically constrained tetrafluoropyridylsubstituted fluorophosphorane was heated to 110 o C for 10 h in o-C6H4F2, i.e. 3 and a difluorophosphorane are formed. In our system, a related phosphorane R3P(C5F4N)F is not observed, as the reaction to form 3 is fast at room temperature. However, the similar reactivity suggests that a fluorophosphorane resulting from OA of 1 to PR3 is a likely intermediate. Looking at the catalytic reaction as a whole, the experimental mechanistic studies suggested that the phosphine catalyst adds to the fluoroarene/heteroarene to initially form a fluorophosphorane, e.g. P(F)( n Bu)3(C5F4N), but that the silane then acts as a Lewis acid to abstract a fluoride ion from the phosphorane to form a phosphonium salt, e.g. [(C5F4N)P n Bu3][Ph2Si(F)H2]. The hydrosilicate anion then transfers a hydride to the phosphonium ion to form another phosphorane that eliminates the hydrodefluorinated product and re-generates the phosphine (Scheme 4). This H/F exchange between P and Si is driven by differences in the fluoride ion and hydride ion affinities of the phosphorane and silane, which have been discussed and computed for related species. The reactivity of non- fluorinated pyridylphosphonium salts with nucleophiles to form substituted pyridines has also been reported and, like the processes observed here, is proposed to occur via phosphorane intermediates. Underpinning all of this reactivity is the facile redox cycling between P(III) and P(V) oxidation states that is remarkable to see in simple trialkylphosphines. DFT calculations were performed to support the experimental mechanistic studies (see SI for details). These showed that initial reaction of the phosphine with pentafluoropyridine takes place through a Meisenheimer-like transition state (Scheme 4, TS12) to form a fluorophosphorane (M2) in a similar manner to that proposed by García and coworkers for reaction of 1 with PEt3. This is associated with a low barrier of 55 kJmol -1 . The structure of TS12 (Figure ) shows the early nature of this transition state, where P-C bond formation and C-F bond elongation precedes fluorine transfer to phosphorus to form the phosphorane. This step is effectively initiated by a nucleophilic addition of PR3 to the pentafluoropyridine and explains the preference for electron-poor arenes/heteroarenes in this reaction, where this will be promoted. No additional intermediates or transition states for fluorine transfer to phosphorus were identified. The addition of 1 to the phosphine leads to a formal oxidation state change from P(III) to P(V) and so can be characterised as an OA process, albeit one that is highly asynchronous in terms of C-F bond cleavage and reminiscent of concerted SNAr mechanisms. The key structural parameters of TS12 are almost identical to those calculated by Dobrovetsky and coworkers for addition of 1 to a geometrically constrained  3 -P compound, suggesting a similar activation process, despite the very different structural frameworks involved. The fluorophosphorane that is initially formed (M2) can undergo isomerisation to a lower energy isomer (M3) with fluorine trans to the tetrafluoropyridyl group, both in the apical positions. Fluoride transfer to Ph2SiH2 results in the formation of the observed phosphonium ion [6] + , in this case as the salt M4. Moving between neutral and ionic manifolds in this way will be strongly influenced by solvation effects. This leads to a small mismatch between the computed energies and the experimental observations, where M4 is higher in energy than the phosphoranes, although phosphonium ions and not the phosphoranes are observed experimentally. This is likely due to limitations in using a dielectric continuum solvation model, which under-solvates the ions and raises their energies relative to neutral species. The neutral/ionic manifold switch may help to explain the solvent effect seen in this system, where moving from o-C6H4F2 ( 13.4 = at 25 o C) to the more polar MeCN (  = 35.9 at 25 o C) led to an increase in the catalytic rate. It is well known that more polar solvents like MeCN promote the formation of ionic species from phosphoranes and this would facilitate the formation of M4.