While we have previously shown that photolysis of Rh(Me)-3 gave rise to reactivity that was consistent with the formation of site-isolated Rh(II), no direct evidence for the formation of Rh(II) was provided. Here we present direct characterization of Rh(II)-3 and Rh(II)-4 via continuous-wave (CW) EPR spectroscopy (Figure ). The shape of the EPR signal arises from the coupling of the electronic spin S=1/2 of Rh(II) with the nuclear spin of 103 Rh I=1/2 with g⊥> ge> g|| typical of a dz 2 singly occupied molecular orbital (SOMO). Computer simulations of the spectrum shows the contribution of a major species with spin Hamiltonian parameters consistent with the reported data for homogeneous porphyrin Rh(II) metalloradicals in a frozen solvent matrix (Figure and). We also derived the average distance between Rh(II) centers in Rh(II)-3 and Rh(II)-4 from line shape analysis of the EPR spectra. By assuming that the broadening of EPR signals results from the interaction between adjacent Rh(II) centers and applying a simple dipolar coupling model (Figure and Table ), we obtained Rh(II)-Rh(II) intermolecular separation values of 1.81 nm in the case of Rh(II)-3 and 1.24 nm for Rh(II)-4. Both values are in good agreement with the separation between porphyrin centers in the X-ray structures of PCN-224 (1.9 nm) and PCN-222 (1.06 nm for triangular pores). A larger discrepancy is expected for PCN-222 due to the presence of hexagonal as well as triangular pores, so that the dipolar coupling observed by EPR is averaged over two sets of Rh(II) centers located at different distances. The agreement between EPR-derived Rh(II)-Rh(II) separations and the crystallographically determined porphyrin spacing is consistent with a complete conversion of Rh(III)Me to Rh(II) sites upon irradiation. Complete conversion of Rh(III)-Me to Rh(II) was further confirmed by 1 H-NMR analysis of the metalloporphyrin linker that was recovered when Rh(II)-3 or Rh(II)-4 were subjected to digestion under basic conditions. We also observed the presence of methane and toluene after Rh(Me)-3 was irradiated in benzene, which is consistent with the release of methyl radicals that undergo reaction with the benzene solvent, as previously described by Wayland and co-workers. Notably, EDX analysis of 3 recovered from catalytic reactions remained consistent with the complete metalation of all porphyrin sites by rhodium. To confirm that the metalloradical species, Rh(II)-3, observed by EPR spectroscopy constitutes a kinetically competent catalyst for olefin hydrosilylation, we monitored the rate of ethylene hydrosilylation (Figure ). Direct use of Rh(Me)-3 in light-promoted hydrosilylation showed an initiation period of close to 5 h, which we attribute to the conversion of the bench-stable Rh(III)-Me precursor to catalytically active Rh(II)-3 (Figure ). Since site-isolation renders the Rh(II) catalyst stable to storage under inert conditions, direct use of Rh(II)-3 in catalysis could also be tested (Figure ). As expected, the initiation period disappeared for Rh(II)-3, and the effective rate constant is identical within experimental error to that observed with Rh(Me)-3 after 5 h of reaction (Figure ). Rh(II)-3 is thus gradually formed under the light-mediated reaction conditions and is a kinetically competent catalyst for olefin hydrosilylation.