Surprisingly, the best fits for the rate (using VTNA or initial rates) had a pseudo-zero order dependence on [Fe]. Using VTNA, there is a deviation at later time points, prompting us to test for catalyst deactivation using a method derived from reaction progress kinetic analysis. Two hydrogenation reactions were conducted in parallel, with the first unchanged from standard conditions. The second reaction started with lower concentration of starting material but included the independently prepared product to mimic a partially complete reaction, but without any catalyst deactivation. Importantly, the two reaction profiles do not overlay (see SI), suggesting that catalyst deactivation occurs during catalysis. This result could help explain the pseudo-zero rate order in catalyst as discussed in our iron speciation exploration below. Perhaps the most salient conclusion from the kinetics, however, is that while the rate is independent of olefin and thiol concentrations, it is dependent on [silane], suggesting that silane is part of the turnover-limiting step. This finding is consistent with mechanistic studies on iron-catalyzed mHAT olefin-olefin cross-coupling, which shares several common intermediates. The slowest step in the coupling reaction is formation of the [Fe III ]-H intermediate, which is high-energy and requires overcoming a high-energy transition state for its formation. This could be the case for the cHAT hydrogenation studied here as well.