Reaction between Os3(CO)12 and but-2-yne gives two complexes tentatively identified as tricarbonyltetramethylcyclopentadienoneosmium and the binuclear complex (MeC2Me)2Os2(CO)6. Cyclo-octatetraene reacts with the carbonyl to give C8H8Os(CO)3, with the hydrocarbon bonded to the metal via a π-allyl group and a carbon–metal σ-bond. On warming, the complex isomerises to a 1,3-diene complex, which exhibits valence tautomerism. This compound reacts with (MeO)3P to give trans-[(MeO)3P]2Os(CO)3. With MePPh2, the carbonyl gives [(MePPh2)Os(CO)3]3. Trimethylsilane and Os3(CO)12 form trans-(Me3Si)2Os(CO)4, in which the Si–Os bonds can be cleaved with hydrogen chloride to give trans-Os(CO)4Cl2. A variety of other halogenocarbonyls resulted from reactions between the carbonyl and CF3I, iodine, or HCl.