The present invention concerns a process for creating a Carbon-Carbon bond (CC) or a Carbon-Heteroatom bond (CHE) by reacting a compound carrying a leaving group with a nucleophilic compound carrying a carbon atom or a heteroatom (HE) that can substitute for the leaving group, creating a CC or CHE bond, wherein the reaction takes place in the presence of an effective quantity of a. a catalytic system comprising a ligand and at least a metal-based catalyst, such a metal catalyst being chosen among iron or copper compounds proviso that only a single metal is present.

An improved catalyst for cyclic olefin polymerization. The catalyst includes a transition metal carbene having the following structure: M.sup.v(OR).sub.c*mX.sub.(v-c*m-2)C(R*).sub.2 wherein M.sup.v is a Group 5 transition metal having a valence (v) of 5 or a Group 6 transition metal having a valence (v) of 5 or 6; each R is independently a monovalent organic moiety comprising from 8 to 40 atoms selected from Groups 14-17; c is an integer from 1 to 3; m is 1/3, 1/2, 1, 3/2, 2, 3, or 4 and c*mv2; X is a halogen; and each R* is independently H or a C.sub.1 to C.sub.7 alkyl. The catalyst is particularly useful for ring-opening metathesis polymerization (ROMP).