Catalyst system comprising transition metal and imidazoline-2-ylidene or imidazolidine-2-ylidene

A composition which comprises

• a) at least one metal compound comprising at least one transition metal atom;
• and
• b) at least one N-heterocyclic carbene selected from the group consisting of
• i) an imidazoline-2-ylidene wherein the 1 and 3 positions are each, independently, substituted by an aromatic group in which each ortho position is, independently, substituted by a secondary or tertiary group which has at least three atoms, or a protonated salt of such an imidazoline-2-ylidene;
• ii) an imidazolidine-2-ylidene wherein the 1 and 3 positions are each, independently, substituted by an aromatic group in which each ortho position is, independently, substituted by a secondary or tertiary group having at least three atoms, or a protonated salt of such an imidazolidine-2-ylidene;
• iii) a bis(imidazoline-2-ylidene) wherein a bridging moiety is bound to one nitrogen atom of each ring, wherein the bridge formed by the bridging moiety has at least five atoms, and wherein the remaining two nitrogen atoms are each, independently, substituted by a secondary or tertiary group which has at least three atoms, or a protonated salt of such a bis(imidazoline-2-ylidene);
• iv) a bis(imidazolidine-2-ylidene) wherein a bridging moiety is bound to one nitrogen atom of each ring, wherein the bridge formed by the bridging moiety has at least five atoms, and wherein the remaining two nitrogen atoms are each, independently, substituted by a secondary or tertiary group which has at least three atoms, or a protonated salt of such a bis(imidazolidine-2-ylidene);
• and mixtures of two or more of the foregoing.