Abstract. [OsBr2(CO)2[P(CtHs)3]2], M r = 930.61, monoclinic, C2/c, a = 24.332 (2), b = 9.568 (2), c = 16.207 (1)/k, /~= 115.882 (6) °, V= 3394.7 (7)/~3, Z=4, Dx=l.82gcm -3, 2(MoK~t)=0.71069/~, /~ = 62.14 cm -~, F(000) = 1800, T = 295 K, R = 0.035, 2994 unique reflections measured, 2186 observed (I > 3aI). Preparation by reaction of bis(tetra-n-butylammonium) hexabromoosmate(IV) with triphenylphosphine in refluxing acetic acid/acetic anhydride. The molecule contains an Os atom in octahedral coordination with three disordered Br/CO atoms/molecules, a single ordered CO molecule and two trans triphenylphosphine groups bonded to Os through their P atoms.Experimental. Schlenk apparatus used for preparation.Mixed solvent prepared by combining acetic acid (20 ml) and acetic anhydride (3 ml) and refluxing under N 2 for 2 h. Bis(tetra-n-butylammonium) hexabromoosmate(IV) (0.8898g, 0.772mmol) and triphenylphosphine (1.7875 g, 6.82 mmol) combined as solids in second flask and evacuated for 30min. Solvent and solids combined and refluxed under N 2 for 18 h. White-yellow crystals washed with CH2CI 2 to obtain white solid and yellow solution. Volume of yellow solution reduced by half to dark-brown solution, hexane layered over solution, brown crystals obtained after 3-4d. Data collected from a nearly orthogonal, 0.30 x 0.19 x 0-16 mm crystal fragment cut from a brown, platy, parent crystal. Rigaku AFC-5S diffractometer, graphite-monochromated MoKa radiation, 09--20 scans, scan speed 10 ° min -~, maximum of three scan repetitions to obtain aF/F< 0.10,