The new cobalt (II) phosphine complex CoCl2((PPrPh2)-Pr-i)(2) was synthesized by reacting CoCl2 with isopropyldiphenylphosphine in ethyl alcohol as solvent. The molecular structure of the complex was determined by the X-ray diffraction method. CoCl2(PiPrPh(2))(2) was then used in combination with methylaluminoxane for the polymerization of 1,3-butadiene: it was found to be highly active and stereospecific for the preparation of 1,2 syndiotactic polybutadiene. The same system was also able to polymerize substituted butadienes giving highly stereoregular 1,2 polymers from E-1,3-pentadiene, 1,3-hexadiene, and 3-methyl-1,3-pentadiene. Some of these polymers are completely new and were never prepared before.