The compound [PMePh3]2Os2Cl8 has been structurally characterized by X-ray crystallography. It is isomorphous with the corresponding rhenium compound and contains an eclipsed Os2Cl82- ion with an Os-Os unit disordered unequally (63/37) in two orientations. A new triply bonded compound [TBA]2Os2I8 has been prepared by treating its chloro analogue with HI gas at 0 °C. Dark brown crystals of composition [TBA]2Os2Ig·CH2Cl2 grown by layering a dichloromethane solution of the compound with toluene have been studied by X-ray crystallography. It contains the Os2I82- ion with a staggered conformation (torsion angle is 46.7°) similar to the Os2X82- ions in [TBA]2Os2X8 (X = Cl, Br). Crystallographic data are as follows. [PMePh3]2Os2Cl8: space group P21/n,a = 10.853 (2) Å, b = 11.893 (3) Å, c = 16.167 (3) Å, β = 93.33 (2)°, V = 2083 (2) Å3, Z = 2. [TBA]2O2I8-CH2C12: space group [formula omited], a = 12.833 (5) Å, b = 20.095 (4) Å, c = 12.331 (3) Å, α = 95.95 (2)°, β = 109.16 (2)°, γ = 106.03 (2)°, V = 2802 (4) Å3, Z = 2. A comparison of important structural dimensions of staggered [Os2X8]2- ions shows that they are completely homologous. © 1990, American Chemical Society. All rights reserved.