During kinetic investigations of thermal olefin hydrosilylation we had to rely on analysis of reaction aliquots because mass transfer limitations were observed when thermal hydrosilylation was carried out inside an NMR tube. Interestingly, even the dissolution of ethylene could become substantially inhibited within narrow vessels. In-situ monitoring of ethylene hydrosilylation reaction catalyzed by Rh(II)-3 led to an abrupt cessation of conversion once the ~0.6 equivalents of ethylene initially dissolved in the reaction solvent were converted, despite the availability of additional ethylene in the headspace of the NMR tube (Figure ). We could confirm ethylene dissolution limitations by EPR, were the concentration of 13 detected was strongly dependent on the extent to which EPR tubes were shaken (Figure ). Determination of reaction orders was thus carried out in standard reaction vials from which aliquots were repeatedly collected for NMR analysis. A change in the olefin substrate from ethylene to 1-pentene was deemed necessary to prevent escape of ethylene during sampling. A catalyst reaction order of one, as well as a silane reaction order of 1.3 could be determined by reaction progress kinetic analysis via graphical overlay developed by Blackmond and Bures (Figure -S81). Reaction order analysis for 1-pentene led to a value of 0.7, but due to the larger variability within data sets collected for 1-pentene, we could only conclude that the 1-pentene order was non-zero and likely ~1 (Figure ). The reaction orders determined are consistent with a rate-limiting transition state that involves Rh(II)-3, silane, as well as olefin. Notably, theoretical calculations suggest that the coordination of an electron-donating substituent such as ethylene or silane in the trans position of the Rh(II) porphyrin leads to a substantial reduction in calculated Gibbs energy of activation. Based on EPR observation of intermediate 13, ethylene was used as an exemplary ligand for all theoretical analyses. However, the experimentally determined 1-pentene order of ~1 excluded 1-pentene coordination in the trans position of Rh(II) during 1-pentene hydrosilylation. We suspect that the increased steric bulk of 1-pentene compared to ethylene could disfavor π-coordination of 1-pentene, so that silane likely occupies the axial coordination site on Rh(II). A silane order of 1.3 is consistent with the participation of more than one silane molecule in the generation of transition state, and theoretical calculations indicate that occupation of the trans position on Rh(II) by silane instead of benzene solvent reduces the reaction barrier by 7.4 kcal•mol -1 (Table ). The slower hydrosilylation rate observed with 1-pentene versus ethylene is consistent with reduced stabilization of a zwitterionic transition structure due to the less efficient electron-donation from a silane compared to an ethylene ligand to Rh(II).