A series of amidino N-heterocyclic carbene ligand supported rare-earth metal bis(alkyl) complexes (2,6-R12C6H3NCR2NCH2CH2(NCHCHN(C6H2Me3-2,4,6)C))Ln(CH2SiMe3)2[R1 = iPr, R2 = Ph, Ln = Lu (1a); R1 = iPr, R2 = Ph, Ln = Sc (1b); R1 = iPr, R2 = tBu, Ln = Lu (2a); R1 = Me, R2 = Ph, Ln = Lu (3a)] were synthesized and characterized by NMR spectroscopy. On activation with [(Ph3C)][(B(C6F5)4)], complexes 1a, 2a and 3a exhibited high activities and high 3,4-selectivities (96.9%) for the polymerization of 2-phenyl-1,3-butadiene in a living mode to afford new plastic polydienes with high glass-transition temperatures (44.3–56.1 °C). The 3,4-selectivity decreased with increasing the polymerization temperature (25–60 °C), but increased with polarity of polymerization solvent in the order of chlorobenzene > toluene > cyclohexane. In contrast, complex 1b was almost inert for the polymerization of 2-phenyl-1,3-butadiene owing to its steric bulkiness.