Thiol Binding to Iron(II). In the iron-catalyzed olefin-olefin cross-coupling via HAT, we proposed that the alkyl radical generated following HAT is quenched via proton-coupled electron transfer (PCET) from an iron(II) ethanol complex, a step which is enabled by the lowering of the O-H BDFE upon coordination of EtOH to the iron center. 17 The cHAT olefin hydrogenation system presents an opportunity to form an analogous iron(II) thiol complex, which would likewise have a decreased S-H BDFE. The feasibility of an iron(II)-thiol interaction comes from literature suggesting that cysteine binds as a neutral donor to iron(II) heme complexes. While Lewis basic ligands with oxygen, nitrogen, and phosphorus donor atoms are known to coordinate to Fe II (acac)2, to the best of our knowledge no studies have yet reported evidence for binding of neutral sulfur-based ligands. 27 Therefore, we sought to experimentally probe whether thiol binds to iron(II) under conditions relevant to olefin hydrogenation.