The tungsten(II) carbonyl compound (CO)(4)W(mu-Cl)(3)W(SnCl3)(CO)(3) has been found to be a very effective catalyst for ditnerization reaction of bicyclo[2.2.1]hept-2-ene to 2-bicyclo[2.2.1]hept-2-ylidenebicyclo[2.2.1]-heptane conducted in dichloromethane solution at room temperature. A very labile eta(2)-norbornene complex of tungsten(II) has been shown to be formed on the pathway to the catalytically active species. A mechanism involving C-H bond activation of the eta(2)-norbornene ligand and the formation of metallacarbene is proposed to explain those results. In addition, it has been determined that other products, such as cyclotrimers and the ring opening metathesis polymerization (ROMP) polymer of norbornene can be formed in amounts depending upon the catalyst-to-norbornene molar ratio. The reaction products are identified by means of chromatography (GC-MS) and H-1 and C-13 NMR spectroscopy. (C) 2004 Elsevier B.V. All rights reserved.