The allenyliclene-ruthenium complexes [(eta(6)-arene)RuCl(=C=C=CR2)(PR'(3))]OTf (R-2 = Ph; fluorene, Ph, Me; PR'3 = PCY3, (PPr3)-Pr-i, PPh3) (OTf = CF3SO3) on protonation with HOTf at -40 degrees C are completely transformed into alkenylcarbyne complexes [(eta(6)-p-cymene)RuCl( CCH=CR2)(PR3)](OTf)(2). At -20 degrees C the latter undergo intramolecular rearrangement of the allenylidene ligand, with release of HOTf, into the indenylidene group in derivatives [(eta(6)-arene)RuCl(indenylidene)(PR3)]OTf. The in situ-prepared indenylidene- ruthenium complexes are efficient catalyst precursors for ring-opening metathesis polymerization of cyclooctene and cyclopentene, reaching turnover frequencies of nearly 300 s(-1) at room temperature. Isolation of these derivatives improves catalytic activity for the ring-closing metathesis of a variety of dienes and enynes. A mechanism based on the initial release of arene ligand and the in situ generation of the active catalytic species RuCI(OTf)(=CH2)(PR3) is proposed.