Various lanthanide aryloxide complexes supported by bridged bis(amidinate) ligand L, LLnOAr(DME) (L = Me₃SiNC(Ph)N(CH₂)₃NC(Ph)NSiMe₃, DME = dimethoxyethane, Ln = Y, Ar = 2,6-(Me)₂C₆H₃ (1), 2,6-(ⁱPr)₂C₆H₃ (2), 2,6-(^tBu)₂-4-(Me)C₆H₂ (3); Ar = 2,6-(^tBu)₂-4-(Me)C₆H₂, Ln = Nd (4), Sm (5), Yb (6)) were synthesized, and complexes 1, 2 and 4–6 were characterized by single crystal X-ray diffraction. All the complexes are efficient precatalysts for catalytic addition of amines to carbodiimides. The catalytic activity is influenced by lanthanide metals and the aryloxide groups (Nd (4) Sm (5) < Y (3) Yb (6) and -2,6-(Me)₂C₆H₃ < -2,6-(ⁱPr)₂C₆H₃ < -2,6-(^tBu)₂-4-(Me)C₆H₂). The catalytic addition reaction with 3 showed a good scope of substrates. The mechanism investigation revealed the real active intermediate being the monoguanidinate complexes supported by an aryloxide and an amidine-functionalized amidinate group, L′Ln[O2,6-(^tBu)₂-4-(Me)C₆H₂][RNCNHRN(Ar′)] (L′ = Me₃SiNHC(Ph)N(CH₂)₃NC(Ph)NSiMe₃, R = ⁱPr, Ar′ = phenyl, Ln = Yb (8), Y (11); R = Cy, Ar′ = phenyl, Ln = Yb (10), Y (12); R = ⁱPr, Ar′ = 4-ClC₆H₄, Ln = Yb (9)), which were isolated from the reactions of 6 (or 3) with amine and carbodiimide in a molar ratio of 1:1:1 and structurally characterized. The Ln-active group in the present precatalyst is a Ln–amidinate species, not the Ln–OAr group.