Near-UV irradiation of (η-C5R′M(CO)3R (R′ = H, Me; M = Mo, W; R = CH3, C2H5, n-pentyl) results in efficient (Φ > 0.1) dissociative loss of CO. If the photoreaction is carried out at 77 K, the 16-valence-electron species resulting from CO loss can be accumulated and characterized by infrared spectroscopy. Relative intensities of the two carbonyl stretching absorptions suggest that the structure of these species is similar to that of the 18-valence-electron (η5-C5H5)Fe(CO)2R, that is, a relaxed structure. Upon warming, (η5-C5R′5)M(CO)2R reacts with CO or added PPh3 to regenerate starting material or form (η5-C5R′5)M(CO)2(PPh3)R, respectively. In the case where R contains β-hydrogens, warming in the absence of ligand results in β-hydrogen transfer to form trans-(η5-C5R′5)M(CO)2(alkene)(H). For the R groups having β-hydrogens, optical and infrared spectroscopy give evidence for a second (η-C5R′5)M(CO)2R species that is not completely coordinatively unsaturated and is proposed to be the immediate precursor to β-hydrogen transfer. This species can be formed by warming or irradiating samples of the photogenerated 16-valence-electron complex and is thought to have a weak M-(β-H) bond. This species does not react rapidly with CO or PPh3 at 195 K as do the (η5-C5R′5)M(CO)2CH3 complexes. The activation energy AG‡ for the conversion of all such 16-valence-electron species to an alkene-hydride is 10 ± 2 kcal/mol, and the C2H5 and C2D5 species react at the same rate. Irradiation of (η5-C5H5)W(CO)3CD2CH3 gives a mixture of trans-(η5-C5H5)W(CO)2(C2H3D)(D) and trans-(η5-C5H5)W(CO)2(C2H2D2)(H) at a temperature where both are inert. The η-hydrogen transfer is proposed to involve a preequilibrium between the 16-valence-electron species and a cis-alkene-hydride complex that isomerizes to the trans alkene-hydride in the rate-determining step. (η5-C5R′5)M(CO)2(alkene)(H) reacts with PPh3 at 298 K to form (η5-C5R′5)M(CO)2(PPh3)(alkyl). For (η5-C5H5)W(CO)2(C5H10)(H) the reaction rate is proportional to PPh3 concentration up to −0.1 M, after which little further increase in rate is observed. The kinetics for this reaction are treated as a preequilibrium between the 16-valence-electron dicarbonyl-alkyl and an 18-valence-electron alkene-hydride with the PPh3 reacting with the dicarbonyl-alkyl. The rate constant for the conversion of the pentene-hydride to the dicarbonyl-pentyl is determined to be 1.7 × 10−3 s−1 at 298 K. In the case of M = Mo, R = C2H5 the equilibrium constant for this interconversion is ∼2 and both species are observed in alkane solution at temperatures as high as 250 K.