Cyclopentadienyl-ruthenium half-sandwich complexes with eta(2)-bound alkyne ligands have been suggested as catalytic intermediates in the early stages of Ru-catalyzed reactions with alkynes. We show that electronically unsaturated complexes of the formula [RuCl(Cp--)(eta(2)-RC[triple bond]CR')] can be stabilized and crystallized by using the sterically demanding cyclopentadienyl ligand Cp-- (Cp--=eta(5)-1-methoxy-2,4-tert-butyl-3-neopentyl-cyclopentadienyl). Furthermore we demonstrate that [RuCl(2)(Cp==)](2) is an active and regioselective catalyst for the [2+2+2] cyclotrimerization of alkynes. The first elementary steps of the reaction of mono(eta(2)-alkyne) complexes containing {RuCl(Cp*)} (Cp*=eta(5)-C(5)Me(5)) and {RuCl(Cp--)} fragments with alkynes were investigated by DFT calculations at the M06/6-31G* level in combination with a continuum solvent model. Theoretical results are able to rationalize and complement the experimental findings. The presence of the sterically demanding Cp-- ligand increases the activation energy required for the formation of the corresponding di(eta(2)-alkyne) complexes, enhancing the initial regioselectivity, but avoiding the evolution of the system towards the expected cyclotrimerization product when bulky substituents are present. Theoretical results also show that the electronic structure and stability of a metallacyclic intermediate is strongly dependent on the nature of the substituents present in the alkyne.