Based on the mechanism proposed for direct silyl radical transfer, a rate acceleration would be expected when the polarity of the solvent is increased: Electron transfer from the adjacent porphyrin center leads to the generation of charges in the transition state, so that a polar solvent environment would be expected to stabilize the transition structure. To ensure that any observed rate differences were not caused by varying ethylene concentrations in solution, we selected a number of solvents for which the experimentally determined ethylene solubility fell into a narrow range (Table ). For n-hexane, benzene, chlorobenzene, and hexafluoroisopropanol (HFIP), the total amount of ethylene present during experiments corresponded to 2.2 -2.5 equivalents relative to silane, of which 0.5 -0.8 equiv were present in solution. Hexane, which showed the highest ethylene solubility but has the lowest dielectric constant (ε = 1.88), gave rise to slower silane conversion (Figure ). On the other hand, replacement of benzene with HFIP (ε = 16.7) increased the conversion to above 80% after a reaction time of 20 h. We also examined ethylene hydrosilylation in HFIP with Rh(II)-3 prepared in the presence of 1 mL of air: complete conversion was observed after 20 h, and even after 5 h, a conversion of 32% had already been reached (Figure ). We attribute the rate enhancement to the formation of a larger number of Rh(III)-O-OH sites that may provide internal solvation to polar transition structures assembled on adjacent Rh(II) centers. The observation of a pronounced oxygen effect in a polar solvent such as HFIP suggests that solvent molecules in the pore cannot fully compensate for the polar stabilization Rh(III)-O-OH moieties provide to the zwitterionic transition state.