Crystals of the title compound are triclinic, space group P, Z= 2, a= 14·395(6), b= 18·895(11), c= 9·451(5)Å, α= 92·53(3), β= 103·25(7), γ= 113·81(5)°. The structure was solved by Patterson and Fourier syntheses, and refined by least-squares methods to R 0·078 for 4053 non-zero reflexions measured on a diffractometer. The molecule is built up by a cluster of three osmium atoms, at the corners of a nearly equilateral triangle, and by two organic ligands; the osmium atoms are bonded to two [Os(1) and Os(3)] or three [Os(2)] carbonyl groups. While the chelation between Ph4C4 and the cluster is attained viaσ-bonds and the donation of π-electrons of the osmacyclopentadiene ring, Ph2C2 appears to be four-electron donor on the whole cluster. The shortest Os–Os bond [2·680(2)Å] faces the Ph4C4 ligand.