The remarkably high efficiency of the pentafluorosulfanylation prompted us to investigate the reaction mechanism and our findings are depicted in Figure (see SI for details). Previous approaches to SF5-compounds are characterized by requiring particularly high temperatures for pentafluorosulfanylation to occur (e.g. 120 °C, AgF, 48 h from Ar-SF4Cl). In sharp contrast, experiments in a shorter reaction time revealed that 2 was already formed in near quantitative yield after 15 min at room temperature (Fig. ). Furthermore, testing various tetraalkylammonium or phosphonium salts showed that besides the corresponding chlorides and bromides, also bench-stable tetraphenylfluorophosphane (PPh4F) was effective (Fig. ). Although the yield was lower, also non-halogen anion, azide (NBu4N3) afforded the desired product. These findings indicate a pentafluorosulfanylation mechanism that does not involve chlorinated or brominated intermediates. Instead, we propose the formation of soluble fluoroargentates(II) from the complexation of Lewis acidic AgF2 by onium salts acting as the active fluorinating species. In NMR experiments, the reaction of PPh4F with AgF2 afforded a red solution with yellow AgF precipitate showing a characteristic 19 F NMR doublet of doublet at -242 ppm (Fig. , left top). We ascribe this signal to a fluoroargentate(II) complex, such as AgF3[X](PPh4)n, where the AgF2 is coordinated by one or more equivalents of fluoride from PPh4F ([X] indicating potential additional ligands). The complex is stable in the presence of an excess of AgF2 for a few hours, but decomposes over time through the fluorination of the solvent CD3CN. The reaction could also be followed by 31 P NMR indicating a broad signal at 14.5 ppm for AgF3[X](PPh4)n and a sharp singlet at 23.3 ppm for PPh4DF2(42) formed after decomposition of the complex (Fig. , right). Upon mixing AgF2 and NEt4Cl, a very similar signal was observed by 19 F NMR indicating that the same fluoroargentate(II) with a different countercation, described as AgF3[X](NEt4)n, was generated through the precipitation of AgCl (Fig. , left bottom). Although fluoroargentates(II) such as KAgF3, K2AgF4 or K3Ag2F7 are known,(43-47) solutionphase fluoroargentate(II) complexes have no precedence in the literature to the best of our knowledge. Consequently, we formulate in Figure a plausible reaction mechanism with PPh4F as the additive. The reaction starts with the fluorination of 1 or other aryl thiol derivatives to an aryl sulfur (IV) trifluorides (Ar-SF3), which even proceeds with AgF2 in the absence of any additive. Due to the nature of silver(II) as a one-electron oxidant, an oxidative fluorine-atom transfer from the fluoroargentate(II) complex would produce aryl tetrafluorosulfur radical intermediates (Ar-SF4•), which have been studied for their stability and propensity to olefin additions. In a second fluorine-atom transfer step, Ar-SF4• would be further fluorinated to the SF5-product. Besides precipitation of AgF, the PPh4F could be regenerated in situ and further coordinate AgF2. Importantly however, the correct stoichiometry of PPh4F and other onium salts is crucial for the full conversion of all intermediates to the SF5-product (Fig. ). Catalytic conditions do not result in high yields and the appropriate amounts also depend on the type of starting material used (see SI for details). The oxidative fluorination is eventually terminated through fluorination of the solvent (observed for CD3CN, MeCN, DCM and others) and the strong tendency to form bifluoride (HF2 -) salts likely prevents further AgF2 complexation.