The complex [Rh(CO),(q5-C,H7)] decarbonylates in refluxing heptane to form [Rh3(p-CO),(q5-C0H7),]. Reaction of the Rh, cluster with ethylene affords a high yield of [Rh(CO)(C,H4)(q5-C,H,)], which reforms the Rh, cluster in solution. Treatment (60 "C, toluene) of [Rh(CO) (C,H,) (q5-C,H7)] with acetylene affords as the major product the bridged vinylalkylidene complex [Rh,(p-C=CH,) (CO),(q5-C,H7),].An X-ray crystallographic study has been carried out on this complex. Crystals are monoclinic, space group P2,/c, Z = 4, in a unit cell of dimensions a = 13.482(3), b = 9.568(3), c = 14.801 (6) A, and p = 100.69(3)". The structure has been refined to R 0.021 (R' 0.021) for 2 999 unique reflections to 28 < 50" (Mo-K,, X-radiation). The p-C=CH, group symmetrically bridges two Rh(CO)(q5-C,H7) fragments linked by a Rh-Rh bond [Rh(l)-Rh(2)2.691(1) A]. A minor product in this reaction is obtained in higher yield by treating acetylene with [Rh,(p-CO),(q5-COH7),], and is characterised as a complex containing two Rh(q5-C9H7) units linked by a p-pentadien-3-one fragment. Reaction of this complex with carbon monoxide or heating affords the cyclopentadienone complex, [ Rh (q4-C,H4CO) (q5-C,H,)]. Reaction of [ Rh,(p-C=CH,) (CO),(q5-C9H,),] with H BF,-Et,O gives [ Rh,(p-CH=CH,) (p-CO) (CO) (q5-CgH7),J [BF,], there being no evidence for the formation of a bridged alkylidyne complex.