Amine elimination of rare-earth-metal tris(silylamide) complexes Ln[N(SiHMe₂)₂]₃(THF)_x (Ln = Sc, x = 1; Ln = Y, x = 2) with 1 equiv. of the amidines [PhC(N-2,6-R₂C₆H₃)₂]H afforded a series of neutral mono(amidinate) rare-earth-metal bis(silylamide) complexes [PhC(N-2,6-R₂C₆H₃)₂]Ln[N(SiHMe₂)₂]₂(THF)_y (R = Me, Ln = Sc, y = 0 (1); R = Me, Ln = Y, y = 1 (2); R = ⁱPr, Ln = Y, y = 1 (3)). Treatment of 1–3 with 1 equiv. of [Ph₃C][B(C₆F₅)₄] in THF generated the corresponding cationic amidinate rare-earth-metal mono(silylamide) complexes [{PhC(N-2,6-R₂C₆H₃)₂}Ln{N(SiHMe₂)₂}(THF)₃][B(C₆F₅)₄] (R = Me, Ln = Sc (4), Y (5); R = ⁱPr, Ln = Y (6)). When 1–3 were first activated with 1 equiv. of [Ph₃C][B(C₆F₅)₄] in toluene, then treatment with THF gave the unexpected cationic amidinate rare-earth-metal amide complexes [{PhC(N-2,6-R₂C₆H₃)₂}LnN{SiHMe₂}{SiMe₂N(SiHMe₂)₂}(THF)_n][B(C₆F₅)₄] (R = Me, Ln = Sc, n = 2 (7); R = Me, Ln = Y, n = 4 (8); R = ⁱPr, Ln = Y, n = 2 (9)). The reaction of 1–3 with excess AlMe₃ produced the heterometallic Ln/Al methyl complexes [PhC(N-2,6-R₂C₆H₃)₂]Ln[(μ-Me)₂AlMe₂]₂ (R = Me, Ln = Sc (10), Y (11); R = ⁱPr, Ln = Y (12)). All these complexes were well-characterized by elemental analysis, NMR spectroscopy and FT-IR spectroscopy. 2, 6 and 11 were further structurally authenticated by X-ray crystallography. The binary catalyst system of 1/[Ph₃C][B(C₆F₅)₄] in toluene showed activity toward 3,4-selective polymerization of isoprene, whilst the tertiary catalyst systems of 1–3/[Ph₃C][B(C₆F₅)₄]/AlMe₃ were highly active for cis-1,4-selective polymerization of isoprene.