Abstract: The present invention describes the use of cationic vinylidene, allenylidene and higher cumulenylidene complexes of ruthenium or osmium as catalysts or catalyst precursors for olefin metathesis reactions of all types, as well as to new cationic allenylidene complexes of ruthenium and osmium which can be used as metathesis catalysts with preferred embodiment. These catalysts or catalyst precursors are easy to prepare from well accessible, stable and essentially non toxic starting materials, can be isolated and stored, they exhibit a high catalytic activity, a good compatibility with functional groups, solvents, water and additives, and they need not to be activated by any additive.