A series of rare-earth metal complexes attached by an amino-functionalized cyclopentadienyl ligand (C₅Me₄H-C₆H₄-o-NMe₂) (1) was prepared. The metathesis reaction of the ligand lithium salt [C₅Me₄-C₆H₄-o-NMe₂]Li with LnCl₃(THF)_n afforded the dichlorido complexes [(C₅Me₄-C₆H₄-o-NMe₂)₂Ln₂Cl₄][LiCl(THF)₂] (Ln = Y (2a), Lu (2b)), which are trinuclear connected by μ₃-Cl and μ₂-Cl multiple bridges. The straightforward metathesis reaction of Ln(BH₄)₃(THF)_n with equimolar [C₅Me₄-C₆H₄-o-NMe₂]Li in THF medium yielded the first linked half sandwich ligand stabilized THF-free rare-earth metal bis(borohydrido) complexes (C₅Me₄-C₆H₄-o-NMe₂)Ln(BH₄)₂ (Ln = Sc (3a), Sm (3b)), respectively. The single component borohydrido complex 3a showed high activity towards the bulk polymerization of methyl methacrylate without specific control, which showed high iso-selectivity (mm = 80%) when the polymerization was performed in benzene medium, and switched to syndio-selectivity (rr = 74% at −20 °C) in polar THF medium, whilst the metal chlorido species was inert. The binary catalyst system of 3a/MgⁿBu₂ had similar catalytic performances when compared with 3a in THF medium, but provided enriched syndio-control in benzene solution that was in contrast to the iso-control of 3a. Surprisingly, 3a upon activation with ⁿBuLi displayed an extremely high activity (1.1 × 10⁶ g mol_Sc⁻¹ h⁻¹) and afforded syndiotactic PMMA (rr = 75%) at low polymerization temperature (−20 °C) in THF.