The tridentate 2,6-bis(benzimidazolyl)pyridine ligands were combined with CoCl2 to prepare a series of complexes of the general formula: [Py(Bm-R)(2)]CoCl2 (where Py = pyridyl, Bm=benzimidazolyl, R=H (3a); -Me (3b); -Bz (3c)). These complexes were characterized by mass and elemental analysis, along with X-ray diffraction analysis. The geometry around the five coordinated Co atom in 3c consists of a distorted square pyramid in which cobalt is surrounded by tridentate NICA ligand and two chlorines. The Co(II) complexes exhibited high catalytic activities for the polymerization of 1.3-butadiene on activation with alkylaluminums to yield predominantly cis-1,4-polybutadiene with high molecular weight. Choice of cocatalysts was found to be a decisive factor in determining the activity and product microstructure. Ethylaluminum sesquichloride (EASC) was found to be the most efficient one resulting in polymers with about 97% 1,4-cis content. The ligand modification by N-alkylation shows considerable influence on the catalytic activity by virtue of variation of electronic environment of the active centers. The stereoselectivity of the catalysts was consistent for a wide range of reaction conditions, except the formation of a considerable amount of 1,4-trans inserted units at high temperature. (C) 2010 Elsevier B.V. All rights reserved.