the invention provides novel organometallic complexes useful as olefin metathesis catalysts . the complexes have an n - heterocyclic carbene ligand and a chelating carbene ligand associated with a group 8 transition metal center . the molecular structure of the complexes can be altered so as to provide a substantial latency period . the complexes are particularly useful in catalyzing ring closing metathesis of acyclic olefins and ring opening metathesis polymerization of cyclic olefins .