A series of bis(amido) complexes of rare earth metals {2-[Ph₂P(O)]C₆H₄NC(tBu)N(2,6-Me₂C₆H₃)}Ln(N(SiMe₃)₂)₂, (Ln = Y (2), Nd (3), La (4)) was synthesized using the amine elimination reaction of Ln[N(SiMe₃)₂]₃ (Ln = Y, Nd, La) and parent amidine 1 in THF, and the products were isolated in 60 (2), 61 (3) and 72% (4) yields, respectively. The X-ray studies revealed that complexes 2–4 are solvent-free and feature intramolecular coordination of the Ph₂PO group to the Ln ion. Complexes 2–4 were investigated as precatalysts for isoprene polymerization. The ternary systems 2–4/borate/AlR₃ (AlR₃ = AliBu₃, AliBu₂H; borate = [Ph₃C][B(C₆F₅)₄], [PhNHMe₂][B(C₆F₅)₄]) were found to be active in isoprene polymerization and enable complete conversion of 1000 equivalents of monomer into polymer at 25 °C within 1–24 h affording polyisoprenes with polydispersities M_w/M_n = 1.12–9.46. The effect of the organoaluminium component and [Ln]/[AlR₃] ratio on the catalytic activity and selectivity of the ternary catalytic systems was investigated. Complexes 2–4 proved to be efficient catalysts for the ring-opening polymerization of rac-lactide, which allow conversion of up to 500 equivalents of monomer into a polymer at room temperature within 30 min and afford atactic polylactides with high molecular weights and moderate molecular-weight distributions (1.29–2.12). Complexes 2–4 appear to be well-suited for achieving immortal polymerization of lactide, through the introduction of large amounts of isopropanol as a chain-transfer agent.