To verify experimentally that the 3c-3e silane σ-adduct adopts an η 1 coordination mode, 6, and its deuterated variant D-6, (Figure ) were interrogated by hyperfine sublevel correlation spectroscopy (HYSCORE). The predicted 1 H hyperfine coupling for the H atom bridging Si and Rh atoms was outside the detection range of HYSCORE experiments at Q-band frequency. However, the gyromagnetic ratio of deuterium, which is about 7 times smaller than that of 1 H, rendered the 2 H hyperfine interaction measurable. The Q-band HYSCORE spectra recorded at a magnetic field corresponding to the gx of both 6 and D-6 exhibit a complex pattern at low frequency in both the (-,+) and (+,+) quadrants due to the interaction of the unpaired electron with N nuclei (I=1) from the porphyrin ring. The correlation peaks are further split into multiplets due to the quadrupole interaction. Therefore single-quantum and nominally forbidden double-quantum transitions are observed in the spectra (Figure ). While the HYSCORE spectrum of 6 was characterized only by the presence of 14 N signals, the spectrum of D-6 also shows cross-peaks centered at the 2 H Larmor frequency (νL = 6.7 MHz) separated by approximately 7 MHz. Measured 2 H hyperfine coupling constants are in good agreement with values calculated (Table ) for the structural model proposed for 6 (Figure ). EPR spectroscopy could also furnish direct evidence for the light-mediated cleavage of silane σ-adduct 6. While 6 was still observed when a sample of it was stored at room temperature for 20 h, subjecting an identical sample to irradiation (390 nm) for 30 minutes led to the disappearance of the signal attributable to 6 (Figure ). Crucially, no Rh(II) signal was observed after irradiation, which is consistent with the silane σ-adduct undergoing HAT, but not with dissociation of 6 into Rh(II) and silane. Theoretical calculations support that HAT from the silane to Rh(II) in the absence of an additional interaction with ethylene is thermally inaccessible at room temperature (ΔG ǂ calc = 39.1 kcal•mol -1 ), but silyl radical release from 6 becomes thermodynamically feasible in the presence of light (the energy of a 390 nm photon corresponds to 73.3 kcal•mol -1 ; see Figures and for detailed discussion). In particular, many of the bright states of 6 were predicted to have significant charge transfer character from the SOMO, which is localized on the Rh-H-Si 3c-3e bond, to the porphyrin ligand (Figure ). The SOMO has bonding character for Si-H and anti-bonding character for Rh-H, so that excitation of one electron from the SOMO weakens the Si-H bond but strengthens the Rh-H bond. Light excitation thus facilitates Si-H bond cleavage, despite the fact that the Si-H bond (BDFE = 28.7 kcal•mol -1 ) is substantially stronger than the Rh-H bond (BDFE = 3.1 kcal•mol -1 ) in the ground state.