Reaction of the low-valent ruthenium complex [κ2-N3]Ru(η6-MeC6H5) (1), [N3] = 2,6-(MesN═CMe)2C5H3N, with acetylene leads to the displacement of toluene and formation of the monoacetylene adduct, [N3]Ru(C2H2) (2). The short alkyne−metal distances in 2 are consistent with 4 e− donation to the metal; that is, there is some degree of overlap of the perpendicular alkyne π- and metal dxz-orbitals. The NMR data and DFT calculations suggest π⊥ donation is weaker in 2 than in many 4 e− donor alkyne complexes. Further reaction of 2 with acetylene leads to catalytic cyclotrimerization and release of benzene, although the catalyst is short-lived. In the case of diphenylacetylene, two molecules of C2Ph2 react per molecule of 1 to generate the metallacyclic [N3]Ru(C4Ph4) (3), which is best described as a ruthenacyclopentadiene, or ruthenole. Compound 3 does not react further with diphenylacetylene, but does react with terminal alkynes by addition of the acetylenic C−H bond across a ruthenole Ru−C bond. The new complexes, [N3]Ru(C≡C-R)(cis,cis-1,2,3,4-tetraphenylbutadienyl-μ-H) (R = H, 5; Ph, 6), contain terminal acetylide and cis,cis-tetraphenylbutadienyl ligands (Ru-(CPh)4H), where the vinylic C−H bond is weakly bound to the metal through an agostic interaction. This type of ruthenole cleavage by terminal acetylenes may explain the short life of 2 as a catalyst for cyclotrimerization. The order in which HCCH and PhCCPh are introduced into the coordination sphere alters the course of the reaction: whereas isolated Ru(C4Ph4) metallacycle 3 is cleaved by acetylene to give 5, preformed acetylene complex 2 reacts with diphenylacetylene to produce the free cyclization product 1,2,3,4-tetraphenylbenzene (and 3). These observations highlight the key role of five-membered metallacycles in alkyne cyclotrimerization, as well as the importance of steric factors in these reactions. Cyclization is observed in cases where the π-system of the ruthenacyclic intermediate is accessible to an incoming alkyne, but not in cases where steric bulk hinders access.