Reactions between Ru(PPh₃)₂(π-C₅H₅)(σ-C₂R₂H)[(I; R = CF₃); (II; R = CO₂Me)] and hexafluorobut-2-yne give Ru(PPh₃)(π-C₅H₅)(C₂R₂·C₄F₆H)[(III; R = CF₃); (IV; R = CO₂Me)]; the crystal structure of (III) shows that it contains a buta-1,3-dienyl group, π-bonded to the metal via the double bond furthest from the carbon–ruthenium σ-bond, and the structure of (IV), as shown by its n.m.r. spectra, is unexpected in that the CF₃C₂CF₃ group has apparently inserted into the C–H bond of (II) rather than into the C–Ru bond.