The protonolysis reaction of β-iminophosphonamine ligand (NPNdipp = Ph2P(NC6H3iPr2-2,6)2) with one equivalent of rare-earth-metal tris(alkyl)s afforded the corresponding bis(alkyl) complexes NPNdippLn(CH2SiMe3)2(THF) (Ln = Sc (1), Lu (2), Y (3), Er (4)). The bis(4-methylbenzyl) complexes NPNdippLn(CH2Ph-4-Me)2(THF) (Ln = Nd (5), La (6)) were prepared by treatment of the tris(4-methylbenzyl) compounds Ln(CH2Ph-4-Me)3(THF)3 with β-iminophosphonamine ligand. The small-size rare-earth-metal-based complexes 1–4 upon activation with AliBu3 and [Ph3C][B(C6F5)4] showed high 3,4-selectivities up to 98.1% for isoprene polymerization. When the larger size rare-earth-metal-based 4-methylbenzyl complexes 5 and 6 were employed instead, moderate 3,4-selectivities were obtained since the opening coordination environment facilitated the 1,4-enchainment (Nd3+: 76.1%; La3+: 62.9%). Replacing AliBu3 by AlEt3, the 5 and 6 systems exhibited high activity and excellent trans-1,4 selectivity for both isoprene (96.5%, 0 °C) and butadiene (92.8%, 20 °C) polymerizations.