Photoreaction of (η5-C5H5)M(CO)2R (M = Fe, Ru; R = CH3, C2H6, n-C5H11) and (η5-C5Me5)Fe(CO)2C2H5 involves dissociative loss of CO as the primary photoprocess following near-uv excitation. The quantum yield for photosubstitution of (η5-C5H5)M(CO)2CH3 is independent of the incoming ligand or its concentration (above 10−3 M); the quantum yield is 0.4 ± 0.05 for M = Ru and 0.7 ± 0.03 for M = Fe. All of the complexes for R having β-hydrogens yield the analogous (η5-C5R′5)M(CO)2H (R′ = H, Me) complexes upon near-uv irradiation at 298 K in fluid, alkane solution. For the (η5-C5H5)Fe(CO)2C2H5 complex the photochemical formation of the hydride occurs even at 77 K in alkane matrices, and there are no infrared detectable intermediates at an extent conversion of <5%. Irradiation of (η5-C5H5)Ru(CO)2C2H5 yields photoreaction at 77 K, initially giving infrared spectral changes consistent with the formation of (η5-C5H5)Ru(CO)(C2H4)(H) followed by spectral changes consistent with the formation of the (η5-C5H5)Ru(CO)2H. Likewise in rigid (paraffin) media at 298 K, initial spectral changes accompanying near-uv irradiation of (η5-C5H5)Ru-(CO)2C2H5 are consistent with the formation of the ethylene-hydride complex. At 298 K, high concentrations of PPh3 suppress the formation of iron-hydride products from irradiation of (η5-C5H5)Fe(CO)2C2H5 in a manner consistent with the intermediacy of (η5-C5H5)Fe(CO)C2H5 from the dissociative loss of CO from the starting dicarbonyl. For the (η5-C5H5)Ru(CO)2CH3, irradiation at 77 K in an alkane matrix yields infrared spectral changes consistent with the formation of (η5-C5H5)Ru(CO)CH3, providing direct evidence for the photogeneration of a 16-valence-electron intermediate. The rate of β-hydrogen transfer for the Fe species created by light is estimated to be >3 X 102 s−1 at 298 K, and 77 K data suggest that the activation energy for β-hydrogen transfer from photogenerated intermediates is less than ∼6 kcal/mol for both the Fe and Ru complexes.