A complete catalytic cycle for the cyclotrimerization of acetylene with the CpRuCl fragment has been proposed and discussed based on DFT/B3LYP calculations, which revealed a couple of uncommon intermediates. The first is a metallacyclopentatriene complex RuCp(Cl)(C4H4) (B), generated through oxidative coupling of two alkyne ligands. It adds another alkyne in 112 fashion to give an alkyne complex (C). No less than three successive intermediates could be located for the subsequent arene formation. The first, an unusual five- and four-membered bicyclic ring system (D), rearranges to a very unsymmetrical metallaheptatetraene complex (E), which in turn provides CpRuCl(eta(2)-C6H6) (F) via a reductive elimination step. The asymmetry of E, including Cp ring slippage, removes the symmetry-forbidden character from this final step. Completion of the cycle is achieved by an exothermic displacement (21.4 kcal mol(-1)) of the arene by two acetylene molecules regenerating A. In addition to acetylene, the reaction of B with ethylene and carbon disulfide, the latter taken as a model for a molecule lacking hydrogen atoms, has also been investigated, and several parallels noted. In the case of the coordinated alkene, facile C-C coupling to the a. carbon of the metallacycle is feasible due to an agostic assistance, which tends to counterbalance the reduced degree of unsaturation. Carbon disulfide, on the other hand, does not coordinate to ruthenium, but a C=S bond adds instead directly to the Ru=C bond. The final products of the reactions of B with acetylene, ethylene, and carbon disulfide are, respectively, benzene, cyclohexadiene, and thiopyrane-2-thione, the activation energies being lower for acetylene.