The present invention provides a process for preparing a metal carbene complex of formula (I): wherein M is a transition metal atom selected from the group consisting of ruthenium, rhodium, iron , cobalt, osmium and iridium; L denotes neutral donor ligands ligated to said metal, such groups being the same or different; X is an anionic ligand; R1 and R2 are each independently selected from the group consisting of hydrogen, substituted or unsubstituted C1-C20 alkyl, substituted or unsubstituted C2-C20 alkenyl, substituted or unsubstituted cycloalkyl, substituted or unsubstituted heterocycloalkyl, substituted or unsubstituted aryl, substituted or unsubstituted heteroaryl, substituted or unsubstituted vinyl, a is 2 or 3 and b is 1 or 2, such process comprising reacting a sulfur ylide of the formula Ar 2 S=CR 1 R 2 or its precursor with an appropriate metal complex comprising a transition metal atom selected from the group consisting of ruthenium, rhodium, iron, cobalt, osmium and iridium, said metal complex being also in dimeric form, at a temperature between +80 °C and -80 °C, in an inert solvent and under inert atmosphere.