Cobalt complexes 1-4 bearing NO-chelate ligands based on condensation products of 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone with aniline, o-methylaniline, alpha-naphthylamine, and p-nitroaniline, respectively, were synthesized, and the structures of 1 and 4 were characterized by single-crystal X-ray diffraction analyses. The bis(beta-ketoamine) cobalt complexes could act as moderately active catalyst precursors for norbornene polymerization with the activation of methylaluminoxane. This catalytic reaction proceeded mainly through a vinyl-type polymerization mechanism. H-1 NMR and IR showed that in all cases, a small amount of double bonds raised from ring-opening metathesis polymerization (ROMP) was present in the polymerization products. The variation of the polymerization conditions affected the ROMP unit ratio in the polynorbornenes. (c) 2005 Wiley Periodicals, Inc.