Since the replacement of ethylene with a non-gaseous olefin led to noticeable reduction in the reaction rate, we determined the experimental Gibbs energy of activation for the hydrosilylation of ethylene itself to permit comparison between experimental and computational data. It was practically challenging to obtain temperature dependent kinetic data for a reaction that requires a solid catalyst, liquid as well as gaseous reagents, and for which the oxygen concentration has a substantial effect on the reaction rate, so that the quality of the data obtained was only modest (Table -31). Nonetheless, experimental activation parameters of ΔG ǂ exp (298 K) = 19.4 ± 0.6 kcal•mol -1 , and ΔH ǂ exp = 17.6 ± 2.7 kcal•mol -1 could be extracted from an Eyring analysis (Table -33, see section 21.2 in SI). To consider the possibility that ethylene may serve as the trans ligand for Rh(II) in ethylene hydrosilylation, we calculated ΔG ǂ exp both for an olefin order of 1 (the experimentally determined value for 1-pentene) as well as 2 (based on the theoretical proposal for ethylene), which led to values of ΔG ǂ exp = 20.0 kcal•mol -1 (ethylene order = 1) and ΔG ǂ exp = 18.8 kcal•mol -1 (ethylene order = 2), respectively. Consequently, even the upper end of the error range for ΔG ǂ exp is substantially below the minimum barrier required to promote hydrosilylation via silyl radical intermediates. It must be stated, however, that direct comparison of the numerical values for the barriers does not provide immediate information on the practical feasibility of the two reaction pathways due to a difference in reaction orders. Unlike the rate of olefin hydrosilylation via the intermediacy of silyl radicals, which would not be expected to depend on the olefin concentration, direct silyl transfer to olefins has a non-zero olefin order, so that low concentrations of olefin could lead to a low reaction rate even for low values of ΔG ǂ exp.