Identifying the Catalyst Deactivation Pathway. In order to assess the nature of catalyst deactivation as identified in the kinetics, we examined the iron speciation during the catalytic reaction via 57 Fe Mössbauer spectroscopy. Solutions flash-frozen during catalysis (at approximately 45% conversion) using 57 Fe(acac)3 showed that the majority of iron in the reaction was present as Fe II (acac)2 (Figure ). We considered that iron reduction could arise from homolysis of iron(III)-hydride and/or iron(III)-alkyl bonds, 8b, 8h, 17 or from reduction by thiol. Examples from the literature abound for reduction of iron(III) by thiols, 25 and we sought to understand whether thiol plays a role in the formation of Fe II (acac)2 in cHAT olefin hydrogenation. Therefore, we treated Fe III (acac)3 with 10 equiv PhSH, which resulted in rapid, complete consumption of Fe III (acac)3 with formation of Fe II (acac)2 and PhSSPh, as identified by Mössbauer and 1 H NMR spectroscopies (Figure ). Thus, the added thiol is a feasible culprit that leads to the inactive Fe II (acac)2 observed in the hydrogenation reaction.