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The polymerization of butadiene‐1,3 with π‐allylic nickel complexes has been studied. π‐Allyl(crotyl) nickel halides themselves are shown to initiate stereospecific polymerization. The stereospecificity of the catalysts is determined by the nature of halogen bonded to metal. In the presence of chlorine complexes cis‐1,4‐polybutadienes are obtained, the iodine ones giving trans‐1,4‐polybutadienes. Lewis acids change the stereospecificity of π‐crotyl nickel bromide and iodide, favoring cis‐1,4‐structure formation. The polymerization of butadiene with rhodium compounds has been investigated. The experimental evidence indicates that the polymerization involves π‐allylic complexes. An attempt is made to explain the mechanism of polymer chain unit formation on the basis of π‐allylic structure of active sites in diene polymerization by transition metal compounds.