Catalysts for metathesis reactions

A ruthenium- or osmium carbene complex catalyst having the general formulae (IIa),

• where
• M is ruthenium or osmium,
• X1 and X2 are identical or different and are two ligands,
• L1 and L2 are identical or different ligands, where L2 can alternatively also be bridged by the radical R8,
• n is 0,
• R1-R8 are identical or different and are each hydrogen, halogen, hydroxyl, aldehyde, keto, thiol, CF3, nitro, nitroso, cyano, thiocyano, isocyanato, carbodiimide, carbamate, thiocarbamate, dithiocarbamate, amino, amido, imino, silyl, sulphonate (—SO3−), —OSO3−, —PO3− or OPO3− or alkyl, cycloalkyl, alkenyl, alkynyl, aryl, carboxylate, alkoxy, alkenyloxy, alkynyloxy, aryloxy, alkoxycarbonyl, alkylamino, alkylthio, arylthio, alkylsulphonyl, alkylsulphinyl, dialkylamino, alkylsilyl or alkoxysilyl, where these radicals may each optionally be substituted by one or more alkyl, halogen, alkoxy, aryl or heteroaryl radicals or alternatively two directly adjacent radicals from the group R1-R8 together with the ring carbons to which they are bound form, by bridging, a cyclic group, preferably an aromatic system, or alternatively R8 can be bridged to another ligand of the ruthenium- or osmium-carbene complex catalyst
• m is 0 or 1, and
• A is oxygen, sulphur, C(R9R10), N—R11, —C(R12)═C(R13)—, —C(R12)(R14)—C(R13)(R15)—, where R9-R15 are identical or different and each have one of the meanings of the radicals R1-R8.