The structures of the first insertion products (syn/anti, cis/trans) in the ring-opening metathesis polymerization (ROMP) of norbornene derivatives using both neutral and cationic molybdenum imido alkylidene N-heterocyclic carbene (NHC) complexes based on an O-chelating NHC, i.e., [Mo(N-2,6-Me-2-C6H3)(N-nnesityl-N'-2-O-1-C6H4-imidazo-lin-2-ylidene)(CHCMe2Ph)(OTf)] (1), [Mo (N-2,6-Pr-2-C6H3) (N-2-,6-'Pr-2-C6H3N'-2-O-1-C6H4-imidazolin-2-ylidene)-(CHCMe2Ph)(OTf)] (2), and [Mo(N-2,6-'Pr-2-C6H3)(N-2,6'-C6H3-N'-2-O-1-C6H4-imidazolin-2-ylidene) (CH3CN)(CHMe2Ph) (B(Ar-F)(4)(-))] (3), have been identified. Also, syn/anti interconversion rates of catalysts 1-3 have been determined in acetonitrile. Correlation of these values with the rate constants of polymorization revealed the importance of a balanced ratio between these two values. Disrupting that balance by changing the solvent or the monomer or by switching to a similar, but more ROMP-active catalyst leads to significant changes in the cis/trans contents of the resulting polymers. Despite the chelating and bulky nature of the ligands, a mechanism that entails inversion at the metal center of the catalyst during polymerization is proposed. Thus, highly cis-syndiotactic ring-opening metathesis polymerization-derived polymers have been prepared with the aid of 3. On the basis of our results, we propose a comprehensive mechanism for the formation of cis- and trans-configured polymers with molybdenum imido complexes containing an O-chelating