Plausible [Fe III ]-Thiolate Complex. During these efforts to identify catalyst deactivation, we also observed an unknown iron(III) species in the Mössbauer spectra of both the olefin hydrogenation and the iron reduction reaction (Figures and). We hypothesized that this unknown iron species could be an iron(III)thiolate complex. As noted at the outset, the plausible interaction of the Fe-based catalysts with the thiol/thiyl catalyst represents one of the most intriguing aspects of thiol cHAT. Attempts to isolate and crystallize the complex were unsuccessful. Therefore, we turned to density-functional theory (DFT) calculations to compute Mössbauer parameters for plausible iron(III)-thiolate complexes, for comparison to Mössbauer spectra of the mixtures generated in situ. We found that the calculated Mössbauer parameters for various iron(III)-thiolate complexes, including monomers and bridging diiron complexes, are similar to one another and all consistent with the experimental values of the unknown complex (see SI). Thus, the precise structure is not clear from these spectroscopic measurements, but these studies support the feasibility of iron-thiolate species.