Two neodymium alkylborohydride complexes that differ with respect to the number of hydrides on the boron center were synthesized and characterized. The μ₂-ligated complex exhibits Nd–B bond lengths that are approximately 10% longer than those in previously reported μ₃-ligated borohydride complexes. For the μ₃-alkylborohydride complex, the Nd–H–B stretching bands observed by IR spectroscopy suggested stronger borohydride coordination than that of [BH₄]⁻, which is probably due to the electron donation of the alkyl groups. These newly prepared complexes successfully promoted trans-1,4 specific polymerization of conjugate dienes upon activation by Bu₂Mg, whereas activation of these complexes by Bu₂Mg/MMAO gave cis-1,4 regular polymer. The trans-1,4 defect of the polyisoprene obtained from the Nd μ₂-alkylborohydride complex/Bu₂Mg/MMAO system (<2%) was much lower than that obtained from previously reported μ₃-BH₄-ligated neodymium complex (11%). Moreover, μ₂-alkylborohydride complex was a good initiator for the polymerization of MMA, probably because of its anionic nature.