Irradiation (λmax = 300-375 nm) of FvRu2(CO)4 (1, Fv = η5:η5-bicyclopentadienyl) or (μ2-η1:η5-cyclopentadienyl)2Ru2 (CO)4 (2) with dimethyl cis- or trans-butenedioate resulted in FvRu2(η2-trans-CHR=CHR)(CO)3 (3, R = CO2CH3). Prolonged irradiation of 1-3 provided FvRu2(η2-trans-CHR=CHR)2(CO)2 (4a, R = CO2CH3) and FvRu2(η2-cis-CHR=CHR)(η2-trans-CHR=CHR) (CO)2 (4b, R = CO2CH3). Photocatalytic isomerization of cis to trans alkene occurred in the presence of 1-4. Irradiation of 1-3 with dimethyl butynedioate produced FvRu2(μ2-η2-dimethyl butynedioate)(CO)3 (5). Prolonged irradiation of 1-5 with the alkyne afforded FvRu2(μ2-η2:η4-CRCRCRCR) 2(CO) (6, R = CO2CH3). Irradiation of a THF solution of 6 generated FvRu2(μ2-η2:η4CRCRCRCR) 2(THF) (7, R = CO2CH3). Photochemical alkyne cyclotrimerization was observed in the presence of 1-7. In the presence of CO, 7 reverted to 6 thermally. Heating 7 in the presence of dimethyl cis-butenedioate, thiophene, PPh3, or dimethyl sulfoxide (DMSO) afforded FvRu2(μ2-η2:η4-CRCRCRCR) 2(L) (8, R = CO2CH3, L = cis-CHR=CHR; 9, L = thiophene; 10, L = PPh3; 11, L = DMSO). Irradiation (300 nm) of a THF solution of 8, 9, or 11 provided 7, while 10 was inert. Thermal conversion of 8 to 10 or 11 was effected only at relatively high temperatures. Treatment of 9 with dimethyl cis-butenedioate, PPh3, or DMSO yielded 8, 10, and 11, respectively. Heating 11 at 210 °C in molten PPh3 afforded slowly 10. Kinetic experiments on the conversion of 9 to 10 point to dissociative substitution, Ea = 30.5 kcal mol-1. Complexes 3-8 have been characterized by X-ray crystal analyses.