Four mono(boratabenzene) rare-earth metal dialkyl complexes, [(3,5-Me₂-C₅H₃BR)Ln(CH₂SiMe₃)₂(THF)] (1: R = NEt₂, Ln = Sc; 2: R = NEt₂, Ln = Lu; 3: R = Ph, Ln = Sc; 4: R = Ph, Ln = Lu), were synthesized efficiently via a one-pot strategy with Li[3,5-Me₂-C₅H₃BR] (R = NEt₂, Ph), LnCl₃(THF)_x (Ln = Sc, x = 3; Ln = Lu, x = 0), and LiCH₂SiMe₃. The solid-state structures of 1 and 2 were determined by single-crystal X-ray diffraction. Variable-temperature NMR studies indicated that the energy barrier for the rotation of aminoboratabenzene in 1 (ΔG^‡ ≈ 71 kJ mol⁻¹) is higher than that of phenylboratabenzene in 3 (ΔG^‡ ≈ 59 kJ mol⁻¹). These mono(boratabenzene) rare-earth metal dialkyl complexes’ catalytic behaviors for styrene polymerization were investigated, and found that mono(boratabenzene) scandium dialkyl complexes show high catalytic activities for syndiotactic polymerization upon activation with cocatalysts.