A new class of air stable and structurally well-defined cobalt complexes with unsymmetrical pincer type ligands ([2-(ArNCMe)-6-(Py)C5H3N]CoCl2) (Ar=C6H5, Py=pyrazol-1-yl, 5a; Ar=2,4,6-Me3C6H2, Py=pyrazol-1-yl, 5b; Ar=2,6-iPr2C6H3, Py=pyrazol-1-yl, 5c; Ar=C6H5, Py=3,5-Me2pyrazol-1-yl, 5d; Ar=2,4,6-Me3C6H2, Py=3,5-Me2pyrazol-1-yl, 5e; Ar=2,6-iPr2C6H3, Py=3,5-Me2pyrazol-1-yl, 5f; Ar=2,6-iPr2C6H3, Py=3,5-iPr2pyrazol-1-yl, 5g and [2-(OCMe)-6-(3,5-diphenylpyrazol-1-yl)C5H3N]CoCl25h) 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.