Reaction of [Os₅(CO)₁₉] with acetylenes: The X-ray crystal structure of [Os₅(CO)₁₇(HCCH)]

The carbonyl cluster [Os₅(CO)₁₉] reacts with acetylene in heptane, under reflux, to give [Os₅(CO)₁₇(HCCH)] (1) and the known complex [Os₄(CO)₁₂(HCCH)] (2) as the major products. The cluster (1) crystallises in the monoclinic space group P2₁/c with a = 20.255(11), b = 9.391(7), c = 28.227(15) Å, β = 91.62(3)°, and Z = 8. The structure was solved by a combination of direct methods and Fourier difference syntheses, and refined by blocked-cascade least squares to R = 0.060 and R′ = 0.060 for 5 496 unique diffractometer data. In the two independent, but structurally similar molecules the Os atom skeleton consists of two triangles sharing a common vertex to give the 'bow-tie' conformation. The acetylene ligand caps one of the triangles, and is σ-bonded to the Os atom involved in bonding in the other triangle and to one of the peripheral Os atoms, and π-bonded to the other peripheral metal atom. The complex [Os₅(CO)₁₉] also reacts with mono- and di-substituted acetylenes. With HCCR (R = Me or Ph), in heptane, the major products were characterised as [Os₅(CO)₁₇(HCCR)] [R = Me (3) or Ph (4)] and [Os₄(CO)₁₂(HCCR)] [R = Me (5) or Ph (6)]. With R¹CCR² [R¹ = R² = Ph or McO(O)C] decarbonylation to [Os₅(CO)₁₆] occurred before there was any reaction with the acetylene.