A new class of air stable and structurally well-defined cobalt complexes with unsymmetrical pincer type ligands ([2-(ArN=CMe)-6-(PY)C5H3N]CoCl2) (Ar = -C6H5, PY = pyrazol-1-yl, 5a; Ar = 2,4,6-Me3C6H2 Py=pyrazol-1-yl, 5b; Ar = 2,6-(Pr2C6H3)-Pr-1. Py = pyrazol-1-yl, 5c; Ar = -C6H5, Py = 3,5-Me(2)pyrazol-1-yl, 5d; Ar=2,4,6-Me3C6H2, Py 3,5-Me(2)pyrazol-1-yl, 5e; Ar = 2,6-(Pr2C6H3)-Pr-i, Py = 3,5-Me(2)pyrazol-1-yl, 5f; Ar = 2,6-(Pr2C6H3)-Pr-i, Py 3,5-(i)Pr(2)pyrazol-1-yl, 5g and [2-(O=CMe)-6-(3,5-diphenylpyrazol-1-y1)C3H3N]CoCl2 5h) were prepared and the molecular structures of 5a, 5c and 5f were determined by single crystal X-ray crystallography. Upon activation by methylaluminoxane (MAO) in toluene at room temperature, all complexes initiate polymerization of 1,3-butadiene (polymer yields: 65-99%), affording polybutadiene with excellent cis-1,4 regularity (97.5-98.7%). The polymer yields and properties in terms of molecular weight and distribution are well controlled by the substituents on iminoaryl rings and pyrazole rings. Selectivity switch from cis-1,4 to syndio-1,2 was also achievable by adding phosphine as microstructure regulator. (C) 2013 Elsevier B.V. All rights reserved.