Treatment of (η5:η5-fulvalene)diruthenium tetracarbonyl [FvRu2(CO)4, 3] with a large excess (8–10 equiv) of PMe3 at 120 °C provides trans -Ru(CO)3(PMe3)2 (7) and FvRu(PMe3)2(CO) (6), in which only one Cp ring is bonded to Ru. Complex 6 was characterized by X-ray crystallography: orthorhombic, a = 1464.3 (4) pm, b = 1535.1 (2) pm, c = 1691.6 (4) pm, space group Pbca. Reaction of 3 with 2–3 equiv of PMe3 or excess (5 equiv) PEt3 gives FvRu2(CO)3L (4, 5). Both substitution products are fluxional on the NMR time scale and a mechanism involving reversible terminal to bridged carbonyl exchange is proposed. Activation parameters for the process were determined from a line-shape analysis of variable-temperature 13C NMR spectra of 13CO-labeled 5.