We embarked on a systematic study to understand why Rh(II) porphyrin centers site isolated in a MOF host were able to promote thermal hydrosilylation of olefins, whereas structurally analogous molecular catalysts proved unreactive. One potential challenge with the molecular catalyst is that the dimerization via Rh-Rh bond formation readily takes place, which depletes the concentration of Rh(II) porphyrin. Preserving a monomeric Rh(II) species in solution is insufficient, however, because no thermal hydrosilylation is observed either with 1, which is sterically shielded from Rh-Rh bond formation. While steric bulk is effective for preventing Rh-Rh bond formation, it offers poor protection for 3c-3e adducts: Even the extreme steric protection offered by four 2,6-diisopropyl phenyl substituents was only able to stabilize a porphyrin Rh(II)-ethylene 3c-3e adduct at low temperatures. The geometric protection offered by the MOF matrix permits the formation of a 3c-3e ethylene π-adduct, and even a 3c-3e silane σ-adduct that both remain stable at room temperature. The combination of stability and steric accessibility of the novel 3c-3e silane σ-adduct 6 permits it to engage with other reaction partners, such as olefins. Preassembly of silane and Rh(II) to form 6 renders it entropically feasible to assemble a tri-component transition state in which the C-Si bond is formed in concert with formation of the Rh-H bond and cleavage of the H-Si bond. For a molecular complex in solution, a corresponding tri-component transition state is entropically inaccessible because no 3c-3e silane σ-adduct adduct can be generated. Site-isolation alone is however insufficient to enable facile direct silyl radical transfer: We found that an electron transfer that takes place as the system approaches the transition state is necessary to enable olefin hydrosilylation to take place at room temperature. While isolated Rh(II) centers could potentially be obtained by attachment of suitably functionalized Rh(II) porphyrin molecules to a support surface, the high degree of spatial organization provided by the MOF host is crucial for ensuring that a Rh(II) center is sufficiently close to a transition structure to engage in facile electron transfer. Additionally, we also found evidence for an unexpected stabilization of the transition structure due to the formation of Rh(III)-O-OH during photolysis of the Rh(Me) precatalyst. Small amounts of O2 exert a pronounced beneficial effect on thermal ethylene hydrosilylation, which we rationalized by considering that the co-generation of minor amounts of Rh(III)-O-OH alongside Rh(II) would position polar groups in close spatial proximity to the active site. The promotional effect of Rh(III)-O-OH remained pronounced when benzene was replaced by HFIP as the reaction solvent, which suggests the effect of proximally located Rh(III)-O-OH is not trivial to recreate via variation of the solvent environment. Notably, both the observation of substantial rate increases in the presence of a polar solvent, and an increase in the formation of the dehydrosilylated reaction products support that electron transfer from an adjacent rhodium center occurs during thermal hydrosilylation.