However, no new species were observed by EPR spectroscopy when Rh(II)-3 was generated in the presence of ~10 instead of ~1 equivalents of O2, either for the active catalyst itself or in the presence of added reagents (Figure ). The intensities of the signals corresponding to Rh(II)-3, 6, and 13 decreased with an increase in the amount of O2 present during Rh(Me)-3 photolysis, however, due to competing formation of Rh(III)-superoxide 9. For a sample of Rh(Me)-3 that was subjected to photolysis under air, all Rh(II)-derived EPR signals were barely detectable, and only 6% conversion was achieved in thermal ethylene hydrosilylation (Figure and). The observation of a sharp decrease in catalyst activity when Rh(II)-3 was generated in air is consistent with Rh(II) centers directly participating in the assembly of the transition state structure.