The reaction of norbornene (NBE) and norbornadiene (NBD) in the presence of seven-coordinate tungsten(II) and molybdenum(II) complexes of the [(CO)(4)M(mu-Cl)(3)M(SnCl3)(CO)(3)] and [MCl(M'Cl-3)(CO)(3)(NCMe)(2)] (M = W, MO; M'= Sn, Ge) types leads to ring-opening metathesis polymerization (ROMP) and to the formation of high molecular weight polymers. The geometric structure of these polymers was determined by means of H-1- and C-13-NMR spectroscopy. The monitoring of the reaction between cyclic olefins and the metal complex by means of H-1-NMR spectroscopy allowed us to observe the coordination of NBD to metal atoms in the initiation step of the polymerization process. Compounds of the [MCl(SnCl3)(CO)(3)(eta(4)-NBD)] type prepared directly from [(CO)(4)M(mu-Cl)(3)M(SnCl3)(CO)(3)] or [MCl(M'Cl-3)(CO)(3)(NCMe)(2)] (M = W, Mo) in the presence of an excess of NBD initiate the ROMP reaction immediately. The detection of the first-formed products in the reaction between the metal complex and cyclic olefins provides valuable information concerning the nature of the initiating species. (C) 2002 Elsevier Science B.V All rights reserved.