A new class of nickel catalysts for stereospecific polymerization of butadiene Ni(COD)₂/[X][B(C₆F₅)₄] (X = CPh₃ or HNMe₂Ph) was investigated and compared to the conventional ternary system Ni(O₂CC₇H₁₅)₂/BF₃·OEt₂/AlEt₃. Both families of catalysts showed high activity and stereospecificity but catalysts based on the relatively non-coordinating anion B(C₆F₅)₄⁻ displayed lower molar masses. The combinations of Ni(O₂CR)₂ with AlEt₃/[CPh₃][B(C₆F₅)₄] and of Ni(COD)₂ with BF₃·OEt₂, [CPh₃][B(C₆F₅)₄]/BF₃·OEt₂ and AlEt₃/BF₃·OEt₂ were also implemented. The polymerization tests using the resulting catalysts demonstrated that in the case of the industrial catalyst Ni(O₂CR)₂/BF₃·OEt₂/AlEt₃, the formation of the active species is based on the reduction of Ni(O₂CR)₂ in the presence of AlEt₃ followed by the re-oxidation of nickel(0) to nickel(II) by butadiene leading to the bis(allyl) complex Ni(C₁₂H₁₈). A cationic active species is finally obtained by the activation of Ni(C₁₂H₁₈) with a fluorinated compound formed by the reaction of BF₃·OEt₂ with AlEt₃.