Catalytic 1,3-butadiene polymerization systems can be obtained with the new complex [NbO(C16H11O6-)(C2O42-)] as a catalyst. When the catalysis occurs in a homogeneous medium (toluene as a solvent) in conjunction with AlEt2Cl, cis-1,4-polybutadiene is obtained. A remarkable inversion of stereoselectivity has been observed when the catalysis is carried out in a two phase system with the molten salt AlCl3.1-methyl-3-n-butylimidazolium chloride (MBIC) as a solvent and AlEt2Cl as a cocatalyst, giving the trans 1,4-polybutadiene.