After analysis of the interaction of reagents with the Rh(II) center we turned our attention to the detection of the proposed reaction intermediates. Since σ-adduct 6 is a stable species in the absence of light and ethylene, silyl radical 8 was only an expected intermediate for the light-mediated hydrosilylation mechanism, while the formation of carboncentered radical 7 was expected for both dark and the lightmediated reaction pathway (Figure ). Indeed, addition of 1 equivalent of TEMPO to the light-mediated hydrosilylation reaction permitted the trapping of both suspected key intermediates, 7 and 8, as 16 and 17 (Figure and S33-S34). TEMPO trapping experiments rely on the capture of fleeting radicals by a persistent radical. A persistent radical that is always present under the reaction conditions is Rh(II)-3, so we wondered if Rh(II) could be used directly as a radical trap, which would minimize the perturbation of the system. A particular concern was that TEMPO may not only be able to capture free silyl radical 8, but potentially also abstract a silyl radical equivalent from 6 (Figure ). Reliance on Rh(II) as a radical trap has the advantage that Rh(II) centers remain immobilized within the MOF framework and are unable to approach 6, while free silyl radical 8 could re-act with Rh(II) centers. Only a very minor portion of the organic radicals could potentially undergo radical-radical recombination with Rh(II) (an irreversible process under thermal conditions), because efficient thermal turnover of the Rh(II) catalyst was observed. To enable a sufficient amount of radical trapping products to accumulate to permit detection, we thus carried out Rh(II)-catalyzed thermal hydrosilylation for two days before the MOF was recovered and analyzed by solid-state Si NMR spectroscopy. A signal for 15, the product expected for the trapping of 7 by Rh(II), and to a lesser extent also 14, the trapping product of 8 by Rh(II), could be observed (Figure ). An approximate quantification is possible via comparison of the intensity of the signals of 15 and 14 with the signal at 7.7 ppm (marked with an asterisk in Figure ), which we attribute to nodebound silanol species. Silanol was formed as minor (~ 0.5%) by-product in all hydrosilylation reactions. The exact amount of silanol observed depends on the level of water and oxygen contamination of the hydrosilylation reaction (Figure ). The chemical shift observed for the additional peak in Figure is consistent with both Zr-O-SiMeEt2 and Et2MeSi-O-SiMeEt2, but because the species in question was not removed by washing of the MOF prior to NMR analysis, we tentatively assign the peak to silanol interacting with the MOF nodes of 3. Since a silanol-derived signal constituted the most intense feature in the solid-state Si NMR spectra of recovered catalysts, only a minute fraction of the silicon-centered and carbon-centered radicals were trapped by Rh(II) centers in the MOF. Because the detection of Rh-[Si] 14 alongside Rh-CH2-CH2-[Si] 15 in catalyst retrieved from thermal hydrosilylation was surprising, further comparative experiments were carried to evaluate a potential contribution of silyl radicals to thermal hydrosilylation. Addition of 8 to Rh(II) could be discerned by solid-state Si NMR analysis of recovered 3 from light-mediated hydrosilylation (14 in Figure ). When Rh(III)Me-3 was irradiated in the presence of only silane, Rh-[Si] 14 could again be detected (Figure ), which substantiates that light-mediated silyl radical release from 6 was possible even in the absence of olefin. For both experiments under light-mediated reaction conditions, a reaction time of 1 h led to the accumulation of a larger amount of Rh-[Si] 14 than was observed in thermal hydrosilylation after 2 d, even though the formation of the Rh-[Si] bond in 14 is reversible in the presence of light. Solid-state NMR data thus indicated that the concentration of silyl radicals is substantially smaller under thermal conditions versus photochemical conditions. Trapping of carbon-centered radical 7, on the other hand, was only observed for thermal hydrosilylation. To gain further insight into the reaction conditiondependent contribution of silyl radicals, we performed double-label experiments with DSiEt3 and HSiMeEt2 (Figure ). Analysis of residual silane after a reaction time of 23 h in the absence of light furnished a Et3SiD : Et2MeSiD ratio of 4.5 : 1, while a 1 : 1 ratio was observed for photochemical conditions. Since a 1:1 ratio of Et3SiD and Et2MeSiD was already established after a reaction time of only 0.5 h in lightmediated hydrosilylation, we could conclude that H/D exchange proceeded very efficiently in the presence of silyl radicals. The limited H/D exchange observed under thermal reaction conditions thus indicates that silyl radicals are only formed in minute amounts under thermal hydrosilylation conditions while direct formation of 7 constitutes the dominant pathway in the dark.