Ethylene on reaction with Os,(CO),, heated under reflux in octane undergoes 1 ,I -elimination of H, to give the ethan-I -yl-Z-ylidyne complex [H,0s3(CH2C) (CO),], (I), whereas cyclopentene and benzene undergo 1.2-elimination of H, to give corresponding cyclopenta-1.2-diylidene and cyclohexa-3.5-dien-I ,2-diylidene complexes ( 11) and (111). Kinetic data are given for intramolecular exchange of the -CH,C< hydrogen atoms of complex (I)which is faster than hydride exchange, and mechanisms for these processes are discussed. Diphenyl(o-vinyl-pheny1)phosphine (dpvp) reacts with Os3(CO),, to give the complex [H,0s3(Ph2P*C6H,.C2H) (CO),], (IV), which is related to the derivatives above.RECENTLY several reactions of cluster carbonyl complexes have been reported in which carbon-hydrogen bond cleavage occurs with olefins to give metal-carbon and -hydrogen bonds. For example, cyclo-octene (C,H1,) reacts with M3(CO)l2 (M = Ru or 0s) to give the complex [H,M,(C8Hl2) (CO),] and bicyclo[3.2.2]octa-2,6-diene (C,H,,) reacts with Ru,(CO),, to give [H,Ru,(C,H,) (CO),] . , The alkene molecule in each case has been dehydrogenated to give a co-ordinated ' alkyne' bridging three metal atoms via one xand two ci-bonds. Hydrogen transfer t o the metal atom has also been reported for reactions of Ru,(CO),, with hexa-2,4-diene or cyclohexa-1,3-diene to give the complex [HRu,(C,H,) (CO),] and with cyclododeca-1,5,9-trienes to give [HRu3(C1,Hl5) (CO)g].5 In these complexes the unsaturated ligand bridges three met a1 atoms via one x-ally1 and two o-bonds.Believing that this type of reactivity is not restricted t o complex alkenes, we have examined reactions of Os,(CO),, with ethylene and cyclopentene, together with the related reaction with benzene. Preliminary reports on some of this work have appeared previously,6*7 Our interest in this type of behaviour started with attempts to prepare the diphenyl(o-vinyl-pheny1)phosphine (dpvp) derivative, [Os(CO),(dpvp)],