In this study, the reactions of the bimetallic compound Na[W₂(µ-Cl)₃Cl₄(THF)₂]·(THF)₃ (1, (W ³ W)⁶⁺, a'²e'⁴) with norbornene (NBE) and some of its derivatives (5-X-2-NBE; X = COOH (NBE–COOH), OH (NBE–OH), CN (NBE–CN), COOMe (NBE–COOMe), CH=CH₂ (VNBE); norbornadiene (NBD)) are described. Complex 1 contains a tungsten–tungsten triple bond, bearing three halide bridges and two labile THF ligands, in a cisoidal relationship along the metal–metal axis. The complex was found to be a highly efficient room temperature homogeneous and heterogeneous unicomponent initiator for the catalytic ring opening metathesis polymerization (ROMP) of most substrates. NBE provides polynorbornene (PNBE) of high molecular weight (M_w) in high yields, soluble in organic solvents. The reaction proceeds with high cis-stereoselectivity (80%–86% cis), independently of the reaction conditions. Strongly coordinating pendant groups (–COOH, –OH, –CN) deactivate 1, whereas substrates bearing softer ones (–COOMe, –CH=CH₂) are quantitatively polymerized. NBD gives quantitatively insoluble PNBD. The polymers have been characterized by ¹H, ¹³C NMR and Size Exclusion Chromatography (SEC). Monitoring the reactions in situ by ¹H NMR (1/NBD or NBE) provides direct evidence of the metathetical nature of the polymerization with the observation of the active tungsten alkylidene propagating polymeric chains. Mechanistic aspects of the reactions are discussed.

First examples of application of dinuclear complexes containing unbrigded metal-metal quadruple bond as catalysts in cyclotrimerization, oligomerization and polymerization of substituted acetylene are reported. The d⁴-d⁴ dinuclear complexes (Bu₄N)₂[Re₂X₈] (1) (1a, X = Cl; 1b, X = Br and 1c, X = I) show low activity and the complex [Mo₂Cl₄(MeOH)₂(PPh₃)₂] (2) medium activity in cyclotrimerization and oligomerization of phenylacetylene (PA). In contrast, the tungsten-based d⁴-d⁴, complex Na₄[W₂Cl₈] (3) exhibits high polymerization activity (polymer yield above 90%) and ca 75% cis-selectivity in tetrahydrofuran. The activity and cis-stereoselectivity of these single-component catalysts decrease in the order 3 >> 2 >> 1, which is proportional to the metal-metal quadruple bond propensity towards ligand addition reactions. Complex 3 induces metathesis polymerization of norbornene (NBE) to poly(cyclopentene-1,3-diylvinylene) with 82% cis-selectivity and copolymerization of PA and NBE to almost all-cis high-molecular-weight copolymer containing cyclopentene-1,3-diylvinylene units, which is free of longer sequences of both PA and NBE units. This suggests that also homopolymerization of PA with 3 proceeds by the metathesis mechanism and that the PA/NBE copolymer structure is close to the alternating one.