While we had gathered various pieces of experimental evidence that the presence of ethylene enables H-Si bond cleavage at room temperature, a transition state model constructed based on the existing data delivered an unrealistically high calculated Gibbs energy of activation (Figure ). Considering that thermal hydrosilylation with Rh(II)-3 can occur at room temperature, it cannot be associated with Gibbs energy of activation that substantially exceeds ~20 kcal•mol -1 . Numerous alternative proposed transition structures also furnished unrealistically high activation barriers or could not be located by our computational approach (Figure ). Since initial models only considered one isolated Rh(II) porphyrin center, we considered that the discrepancy may arise from the promotional effects of the extended environment. Interestingly, when a second Rh(II) porphyrin center was positioned at the distance set by the MOF framework, an electron transfer from the Rh-H-Si-ethylene adduct to the adjacent Rh(II) center could take place. Electron transfer gave rise to a zwitterionic transition state (Figure ) that was slightly lower in energy than the neutral transition state (Figure ). Prior EPR studies had shown however that simultaneous addition of silane and ethylene to Rh(II)-3 led to the formation of bis-adduct 13 (Figure ). Trans-ligation of Rh by an electron-donating ligand would be expected to stabilize the cationic Rh center present in a zwitterionic transition state (Figure ) and thus lower the barrier associated with it. Theoretical models indicated that, while an ethylene adduct is accessible, it is 4.7 kcal•mol -1 less stable than a benzene adduct of 6. However, the increased electron donation provided by ethylene compared to benzene as the trans ligand on Rh(II) led to superior stabilization of the transiently positively charged transition structure. Even taking into account the estimated energy penalty of 4.7 kcal•mol -1 for the formation of 13, the transition state structure depicted in Figure gave rise to the lowest barrier observed thus far (29.7 kcal•mol -1 ). Interestingly, intrinsic reaction coordinate (IRC) analysis showed that the electron transfer from the ethylene adduct of 13 to an adjacent Rh(II) center occurs immediately prior to the transition state (Figure ). Shortly after the barrier is crossed, a second electron transfer takes place which returns both Rh centers to an oxidation state of two (Figure ). While the theoretical model now accounted for the promotional effect of an adjacent rhodium center, it did not reproduce the polarity of the extended environment. Since computation of an entire MOF pore at even the DFT level with a double zeta basis set was too expensive, we accounted for the higher polarity of the MOF pore environment versus a benzene solvent environment via a correction to the dielectric constant in the solvent model (Table ). Use of a dielectric constant consistent with the calculated polarizability of the MOF, as well as complete filling of the MOF's pores by benzene, furnished a theoretical Gibbs energy of activation of 24.9 kcal•mol -1 .