Nine mixed ligand ruthenium(II) vinylidene complexes with the general formula: [RuCl2{=C=CHR'}(PCy3)(L)] and [RuCl{=C=CHR'}(PCy3)(sal-R)] (L = N-heterocyclic carbene, sal-R = salicylaldiminate anion, R' = Ph, SiMe3,But) has been synthesized and characterized. These complexes are easily accessible from [RuCl2(p-cymene)](2), terminal alkynes, imidazolium salts or salicylaldimine salts and they have been found to serve as good catalyst precursors for ring-opening metathesis polymerization (ROMP) of norbornene, substituted norbornenes, polycyclic alkenes and cyclooctene and ring-closing metathesis (RCM) of alpha,omega-dienes. Furthermore, these precursors possess extremely high stability toward air, heat and moisture in comparison with other metathesis-active alkylidene ruthenium systems. No significant catalyst decomposition was found for several days at elevated temperatures. (C) 2003 Elsevier Science B.V. All rights reserved.