A series of ion-pair cobalt complexes (C1-C6) supported by (benzimidazolyl)pyridine alcohol ligands ([NNO]) were synthesized and characterized. The single-crystal X-ray diffraction demonstrated that the ion-pair cobalt complexes formed, consisting of [CoL2](2+) and [CoCl4](2-), in which the cobalt center was surrounded by two [NINO] ligands and adopted distorted octahedronal geometry in the cationic moiety along with [CoCl4](2-) as the counterion. The type of cocatalyst had a remarkable influence on the catalytic properties and ethylaluminum sesquichloride (EASC) was proved to be the most efficient cocatalyst for the polymerization of 1,3-butadiene to produce high conversion of butadiene and high cis-1,4 content in the resulting polymers. Upon activation with EASC, all the cobalt complexes (C1-C6) displayed high catalytic activity for the polymerization of 1,3-butadiene to yield cis-1,4-polybutadiene with high selectivity (>95%) under the Al/Cat molar ratio of 50 at 30 degrees C. The conversion of butadiene, microstructure and properties of the resulting polymers were also affected by the reaction parameters and ligand structure. (C) 2013 Elsevier B.V. All rights reserved.