Broad-band irradn. (lmax = 350 nm) of FvRu2(CO)4 (1, Fv = h5:h5-bicyclopentadienyl) resulted in rapid isomerization to colorless (m2-h1:h5-cyclopentadienyl)2Ru2(CO)4 (2) in a novel process involving a formal dinuclear oxidative addn. to a C-C bond. The product reverted to 1 upon heating in soln. 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 mixt. of 1 and 1-d8 revealed the absence of label scrambling, pointing to intramol. pathways. The quantum yield (0.15) was unaffected by the presence of CCl4, and no chlorination products were obsd. under these conditions. Irradn. of solns. of 1 or 2 with 300 nm light provided Fv(m2-h1:h5-cyclopentadienyl)2Ru4(CO)6 (6) or, in the presence of alkynes, the adducts FvRu2(CO)3(RCCR) (8-10, R = H, C6H5, CO2CH3). Heating 1 and PR3 (R = CH2CH3, CH3, or OCH3) yielded FvRu2(CO)3(PR3) (12-14), in which a fluxional process occurs characterized by intramol. terminal to bridging carbonyl exchange. While 12 and 13 were inert, compd. 14 rapidly and reversibly afforded the P(OCH3)3-substituted analog of 2 (15) upon irradn. 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 stereochem., via the diastereomeric photoproducts 20. A double regiochem. labeling expt. proceeded with retention of connectivity and stereochem. A concerted mechanism for the photoisomerization is consistent with the exptl. observations, but a biradical pathway cannot be ruled out. Kinetic data for the isomerizations of 2, 15, 20a, and 20b to their resp. metal-metal-bonded Fv precursors were detd. 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 expt. with 2-13CO. Kinetic and labeling expts. point to a change in mechanism when the thermal reversion of 2 to 1 was run under CO (.apprx.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 FvRu2(CO)4Cl2 by yet another mechanism. As in the photoisomerization of 1, the thermal reversion of 2 may follow a concerted pathway, although biradical intermediates cannot be excluded.