The cluster compounds Os3(CO)10(μ3-S) (I) and Os3(CO)10(NCMe)2 (II) combine when refluxed in benzene solvent to yield the new cluster compound Os6(CO)19(μ3-S) (III) in 31% yield and the known cluster Os5(CO)15(μ4-S) (IV) in 18% yield. The structure of III was determined by single-crystal X-ray diffraction methods: space group P1, a = 11.119 (2) Å, b = 11.357 (3) Å, c = 12.904 (2) Å, α = 104.51 (2)°, β = 91.12 (1)°, γ = 108.80 (1)°, V = 1484.4 (11) Å3, Z = 2, ρcalcd = 3.82 g/m3. The structure was solved by direct methods and refined with 3847 reflections (F2 ≥ 3.0σ(F2)) to yield the final residuals R1 = 0.038 and R2 = 0.045. The structure consists of a butterfly tetrahedron of four osmium atoms with two additional osmium tetracarbonyl groups bridging adjacent edges of the butterfly tetrahedron. A triply bridging sulfido ligand bridges one of the open triangular faces of the cluster. When refluxed in toluene solvent, III loses 2 mol of CO and is converted into the new cluster compound Os6(CO)17(μ4-S) (V) in 23% yield. The structure of V was also determined by a single-crystal X-ray diffraction analysis: space group Pna21, a = 11.371 (3) Å, b = 16.550 (3) Å, c = 14.140 (3) Å, V = 2661 (2) Å3, Z = 4, ρcalcd = 4.12 g/cm3. The structure was solved by direct methods and refined with 1887 reflections (F2 ≥ 3.0ρ(F2)) to yield the final residuals R1 = 0.029 and R2 = 0.030. The structure consists of a capped square pyramid of six osmium atoms with a quadruply bridging sulfido ligand spanning the base of the square pyramid. The observed structure is in accord with the skeletal electron pair theory, but the metal-metal bonding to the capping group is highly distorted. Both the shortest (2.625 (1) Å) and longest (2.930 (1) Å) metal-metal bonds in the molecule involve the capping group. This can be rationalized by a combination of resonance structures, one of which includes a multiple bond at the shortest bond distance and no bond at the longest distance. © 1984 American Chemical Society.