Broad-band irradiation (lambda(max)=350 nm) of FvRu(2)(CO)(4) (1, Fv=eta(5):eta(5)-bicyclopentadienyl) resulted in rapid isomerization to colorless (mu(2)-eta(1):eta(5)-cyclopentadienyl)(2)Ru-2(CO)(4) (2) in a novel process involving a formal dinuclear oxidative addition to a C-C bond. The product reverted to 1 upon heating in solution or in the solid state, under the latter conditions with an enthalpy change of -29.8 (1.5) kcal mol(-1). Mechanistic studies with a mixture of 1 and 1-d(8) revealed the absence of label scrambling, pointing to intramolecular pathways. The quantum yield (0.15) was unaffected by the presence of CCl4, and no chlorination products were observed under these conditions. Irradiation of solutions of 1 or 2 with 300 nm light provided Fv(mu(2)-eta(1):eta(5)-cyclopentadienyl)(2)Ru-4(CO)(6) (6) or, in the presence of alkynes, the adducts FvRu(2)(CO)(3)(RCCR) (8-10, R = H, C6H5, CO2CH3). Heating 1 and PR3 (R = CH2CH3, CH3, or OCH3) yielded FvRu(2)(CO)(3)(PR3) (12-14), in which a fluxional process occurs characterized by intramolecular terminal to bridging carbonyl exchange. While 12 and 13 were inert, compound 14 rapidly and reversibly afforded the P(OCH3)(3)-substituted analog of 2 (15) upon irradiation with UV light. The two diastereomeric 3,3’-di-tert-butyl-substituted fulvalene analogs of 1 (19) underwent the same reaction sequence with complete retention of stereochemistry, via the diastereomeric photoproducts 20. A double regiochemical labeling experiment proceeded with retention of connectivity and stereochemistry. A concerted mechanism for the photoisomerization is consistent with the experimental observations, but a biradical pathway cannot be ruled out. Kinetic data for the isomerizations of 2, 15, 20a, and 20b to their respective metal-metal-bonded Fv precursors were determined. The entropies of activation (+7 to +21 eu) suggested a disordered transition state. A sequence involving reversible CO loss was ruled out through a crossover experiment with 2-(CO)-C-13. Kinetic and labeling experiments point to a change in mechanism when the thermal reversion of 2 to 1 was run under CO (similar to 1 atm). The occurrence of ligand-induced C-C coupling was indicated through studies of the reactivity of 2 with P(CH3)(3). Photoisomer 2 reacts with excess CCl4 to give FvRu(2)(CO)(4)Cl-2) by yet another mechanism. As in the potoisomerization of 1, the thermal reversion of 2 may follow a concerted pathway, although biradical intermediates cannot be excluded.