The six-coordinate W0 complex cis-W(CO)4(C5H5N)2 has been found to be active in the in situ formation of a carbene species from norbornene, which generates a typical ring-opening metathesis product (ROMP). A proposed mechanism of initiation suggests that the reaction involves a 2,3-hydrogen shift in the coordinated norbornene (η2 → η1). The initiating carbenoid group is identified from the products of the spontaneous carbeneCO coupling and Wittig reactions test. Formation of W(CO)3(η6-C6H5CH3) when toluene is the solvent, followed by reaction with the carbene, is blamed for catalyst deactivation.Key words: bis-pyridine-tetracarbonyl-tungsten, ROMP, olefin metathesis, norbornene.