Irrespective of whether 7 is formed directly from 6 (thermal pathway), or whether hydrosilylation proceeds via lightpromoted expulsion of 8 from 6 (photochemical pathway), however, Rh(III)-H (5) was expected to constitute a key reaction intermediate in both pathways. The ability of C-centered radicals to abstract a hydrogen atom from H-Rh(III) porphyrin complexes was shown already in 1985 by Halpern and coworkers. Rh(III)-H could therefore undergo hydrogen atom abstraction by a carbon-based radical to yield the hydrosilylation product and regenerate Rh(II). We thus set out to gather evidence for the formation of Rh(III)-H under both light-mediated and dark reaction conditions. Solid-state 1 H-NMR analysis of 3 recovered for hydrosilylation reactions under light-promoted or dark conditions both furnished a signal at -43 ppm, which indicates the presence of a hydride ligand trans to a vacant coordination site (Figure ). To provide an unambiguous assignment we also prepared a Rh(III)-H complex derived from the homogeneous model system 18 for the MOF catalyst, the 1 H NMR signal of which was in good agreement with that obtained from recovered 3 (Figure ). We could furthermore show that Rh(III)-H 5 was also formed in the absence of ethylene, so that light-mediated release of silyl radical 8 from 6 is a viable reaction step in light-mediated olefin hydrosilylation. To obtain additional evidence that the proposed intermediated 5 and silyl radical 8 are formed under the reaction conditions, we performed EPR spectroscopy after addition of the spin trap N-tert-butyl-α-phenylnitrone (PBN; Figure ). The formation of three different radical intermediates could be detected upon irradiation of a mixture of Rh(II)-3 and HSiMe2Ph. Based on the respective hyperfine coupling constants of the 1 H nucleus in the α position, we could conclude that PBN adducts of carbon-, silicon-and hydrogen-centered radicals contribute to the experimentally observed signal. A previous report of successful transfer of a hydrogen atom from Au-H to PBN substantiates our assignment that the hydrogen atom trapped by PBN originated from 5. To confirm that a PBN adduct of a silyl radical contributed to the experimentally observed signal we repeated the experiment with HSiEt3 (Figure ), whereupon we observed noticeable changes in the EPR spectrum due to small differences in the Si hyperfine coupling with HSiMe2Ph versus HSiEt3. Molecular Dynamics Simulations Formation of carbon-centered radical intermediate 7, either via addition of 8 to ethylene or direct silyl transfer, is expected to be irreversible: Quantum mechanical calculations show that intermediate 7 is stabilized by ΔGcalc = 15.8 kcal•mol -1 relative to 8 and ethylene. We propose that 7 undergoes HAT with Rh(III)-H 5 to reform Rh(II)-3. Use of deuterated silane confirmed that the deuterium atom originally present in the silane was fully transferred to the hydrosilylation product (Figure ). Molecular dynamics simulations (GFN2-xTB with certain semiempirical parameters re-fitted against DFT data; see Figure ) were able to provide additional insight into the fate of key intermediate 7.