Notably, however, EPR spectroscopy also provided evidence for the presence of an additional thermally accessible pathway: addition of ethylene led to the disappearance of 3c-3e silane adduct 6 within 20 h at room temperature (Figure ). The absence of signals attributable to Rh(II)-3 or its ethylene complex supported the proposal that ethylene addition led to hydrosilylation along with formation of diamagnetic Rh(III)-H. To understand the interactions of Rh(II)-3 with ethylene and silanes in more detail, we introduced ethylene to a benzene suspension of Rh(II)-3, which resulted in the formation of 3c-3e π-adduct 11. Our assignment was based on the excellent agreement of the spectral features of 11 with those reported by Wayland and cowork-ers for the ethylene 3c-3e π-adduct generated from sterically shielded Rh(II) porphyrin 2. Next, we exposed active catalyst Rh(II)-3 dispersed in benzene to both silane and ethylene in the absence of light and immediately froze the EPR sample in liquid nitrogen. The resulting sample contained a new species associated with gx = 2.322 (5), gy = 2.233 (5) and gz = 1.984 (3), in addition to ethylene adduct 11 and silane adduct 6. We subsequently confirmed that the new signal was detected irrespective of the order in which ethylene and silane were added to Rh(II)-3 (Figure ). Having ruled out addition complexes of oxygen, water, or benzene solvent (Table ), we considered that the signal may originate from van der Waals adduct 12 or bis-axial adduct 13 (Figure ). Theoretical calculations estimated a value of 2-4 MHz for the coupling between the unpaired spin and the ethylene protons in 13 (Table ). However, the low signal-to-noise ratio of the echo-detected signal at g = 2.322 position prevented experimental verification of the presence of weak coupling interactions. Theoretical calculations predicted, however, that the association of ethylene in adduct 12 is only fleeting (Figure ), so that spectroscopic observation of 12 was deemed unlikely. We thus tentatively attribute the signal with gx = 2.322, which was only observed when both ethylene and silane were present, to the formation of 13 (Table ). Since we had found that the addition of larger amounts of O2 during the formation of Rh(II)-3 led to an increased activity of the active catalyst during hydrosilylation, we tested whether the increased catalytic activity was associated with the formation of any new radical intermediates.