The present invention relates to novel metathesis catalysts with an imidazolidine-based ligand and to methods for making and using the same. The inventive catalysts are of the formula ##STR00001## wherein: M is ruthenium or osmium; X and X.sup.1 are each independently an anionic ligand; L is a neutral electron donor ligand; and, R, R.sup.1 R.sup.6, R.sup.7, R.sup.8, and R.sup.9 are each independently hydrogen or a substituent selected from the group consisting of C.sub.1-C.sub.20 alkyl, C.sub.2-C.sub.20 alkenyl, C.sub.2-C.sub.20 alkynyl, aryl, C.sub.1-C.sub.20 carboxylate, C.sub.1-C.sub.20 alkoxy, C.sub.2-C.sub.20 alkenyloxy, C.sub.2-C.sub.20 alkynyloxy, aryloxy, C.sub.2-C.sub.20 alkoxycarbonyl, C.sub.1-C.sub.20 alkylthiol, aryl thiol, C.sub.1-C.sub.20 alkylsulfonyl and C.sub.1-C.sub.20 alkylsulfinyl, the substituent optionally substituted with one or more moieties selected from the group consisting of C.sub.1-C.sub.10 alkyl, C.sub.1-C.sub.10 alkoxy, aryl, and a functional group selected from the group consisting of hydroxyl, thiol, thioether, ketone, aldehyde, ester, ether, amine, imine, amide, nitro, carboxylic acid, disulfide, carbonate, isocyanate, carbodiimide, carboalkoxy, carbamate, and halogen. The inclusion of an imidazolidine ligand to the previously described ruthenium or osmium catalysts has been found to dramatically improve the properties of these complexes. The inventive catalysts maintains the functional group tolerance of previously described ruthenium complexes while having enhanced metathesis activity that compares favorably to prior art tungsten and molybdenum systems.