The invention relates to alkylidine N-heterocyclic carbene complexes of general formulas (I) to (VI), wherein A1 represents NR2 or PR2, A2 represents CR 2R2', NR2, PR2, O or S, A3 represents N or P, and C represents a carbene carbon atom, ring B is an unsubstituted or a mono or poly-substituted 5 to 7-membered ring, substituents R2 and R2' represent, inter alia, a linear or branched C1-C18-alkyl group and, if A1 and A2 each represent NR2 or PR2, are the same or different, M in formulas (I) to (VI) represents Mo or W, X1 or X2 in formulas (I) to (VI) are the same or different and represent, inter alia, C1-C18 carboxylates and C1-C18, Z1 and Z2 represent, inter alia, a linear or branched C1-C18-alkylenoxy group, and R1 in formulas (I) to (VI) are, inter alia, an aliphatic or aromatic group. Said compounds are particularly suitable as a catalyst for alkyne metathesis reactions and have, compared to known alkyne metathesis catalysts, the advantage of having a significantly improved activity and stability.
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