Reaction of [Ru₃(CO)₁₂] with 4-methylpent-2-yne yields, as the major product, an open cluster [Ru₃(CO)₈(C₁₂H₁₈)], the result of an unusual type of dimerization and dehydrogenation of an internal alkyne. The crystal structure of [Ru₃(CO)₈(C₁₂H₁₈)] has been determined by X-ray methods. Crystals are triclinic, space group P, with Z= 2 in a unit cell of dimensions a= 9.719(8), b= 13.862(10), c= 9.039(8)Å, α= 103.40(8), β= 103.60(7), and γ= 87.76(6)°. The structure has been solved from diffractometer data by Patterson and Fourier methods and refined by full-matrix least squares to R= 0.033 for 3 701 observed reflections. The complex consists of a bent arrangement of three Ru atoms (with Ru–Ru bonds of 2.661 and 2.645 Å and Ru–Ru–Ru angle of 89.1°) co-ordinated by eight (six terminal and two bridging) carbonyls and by an organic ligand, derived from dimerization and dehydrogenation of 4-methylpent-2-yne. This organic ligand forms two σ bonds with the central Ru atom, giving rise to a metallocyclopentadiene ring in which the terminal Ru atoms are on opposite sides of this ring and are η interacting with its four carbon atoms.