Experimental and computational mechanistic studies were undertaken to probe the pathways underpinning the HDF reaction. Under the optimised reaction conditions for the HDF of 1 (Table , decreased. This suggested that [15] + is an in the catalytic cycle, but that it does not react directly with the silane to form the HDF product. Addition of fluoride to the silane to form a silicate anion, [Ph2Si(F)H2] -, is necessary to promote H -transfer and liberation of the hydrodefluorinated product. Similar silane/silicate reactivity has been reported by Ogoshi and coworkers in metal-free HDF reactions using catalytic [Bu4N][Ph3SiF2] (TBAT). In their case, it was proposed that fluoride transfer from TBAT to silanes, such as Ph3SiH, generated hydrofluorosilicate ions that through ligand redistribution form transient dihydrosilicate ions that undergo HDF of fluoroarenes directly. In the present work, it is not clear whether mono-or dihydrosilicate ions are involved in hydride transfer to phosphonium ions like [15] + . In order to assess the potential role of hydride transfer directly from a hydrosilicate anion to 1, i.e. bypassing the phosphine-induced HDF, a control reaction was run where 1, Ph2SiH2 and 10 mol% of [NMe4]F were allowed to react in MeCN at 20 o C. After 20 minutes some of the HDF product 2 was formed, but only in 19% yield, alongside mostly unreacted 1. After 18 h the same reaction had reached a yield of 64% of 2. This contrasts strongly with the reactivity mediated by P n Bu3 (Table , entry 6) where 93% of 2 was formed under analogous conditions after only 20 minutes.