A series of bis(alkyl) complexes {(tBu)C[N(2,6-Me2C6H3)]2}Ln(CH2SiMe3)2(THF)n (Ln = Y, n = 1 (1); Ln = Sc, n = 1 (2)), {2-[Ph2P(O)]C6H4NC(tBu)N(2,6-Me2C6H3)}Sc(CH2SiMe3)2 (3), {2-[Ph2P(NPh)]C6H4NC(tBu)N(2,6-Me2C6H3)}Sc(CH2SiMe3)2 (4) coordinated by bidentate (N,N) and tridentate (N,N,O; N,N,N) amidinate ligands are synthesized using an alkane elimination approach. Yttrium complex 1 demonstrated a half-life of ∼2.5 days at room temperature in benzene-D6 (C6D6) solution, whereas scandium complexes proved to be much more stable (25 d (2), 30 d (3) and 42 d (4)). Complexes 1-4 as a part of ternary catalytic systems 1-4/TB, HNB/AlR3 (AlR3 = AliBu3, AliBu2H; TB = [Ph3C][B(C6F5)4], HNB = [PhNHMe2][B(C6F5)4]) demonstrated high catalytic activity in isoprene polymerization and enable 80%-100% conversion of 1000 equivalents of monomer into polymer at 25 °C within 3-180 min. The isolated polyisoprenes feature predominantly cis-1,4-regularity (69.2%-87.3%) and polydispersities Mw/Mn = 2.26-8.92. Moreover, the binary (2/TB) and ternary (1-4/TB/10 AliBu3) systems initiate 1-heptene polymerization providing 40%-100% conversion of 500 equivalents of monomer in 24 h at 25 °C giving polymer samples with Mn = 1.55-190.2 × 103 and Mw/Mn = 1.55-3.87.