A series of rare-earth metal complexes attached by an amino-functionalized cyclopentadienyl ligand (C(5)Me(4)H-C(6)H(4)-o-NMe(2)) (1) was prepared. The metathesis reaction of the ligand lithium salt [C(5)Me(4)-C(6)H(4)-o-NMe(2)]Li with LnCl(3)(THF)(n) afforded the dichlorido complexes [(C(5)Me(4)-C(6)H(4)-o-NMe(2))(2)Ln(2)Cl(4)][LiCl(THF)(2)] (Ln = Y (2a), Lu (2b)), which are trinuclear connected by mu(3)-Cl and mu(2)-Cl multiple bridges. The straightforward metathesis reaction of Ln(BH(4))(3)(THF)(n) with equimolar [C(5)Me(4)-C(6)H(4)-o-NMe(2)]Li in THF medium yielded the first linked half sandwich ligand stabilized THF-free rare-earth metal bis(borohydrido) complexes (C(5)Me(4)-C(6)H(4)-o-NMe(2))Ln(BH(4))(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 degrees C) in polar THF medium, whilst the metal chlorido species was inert. The binary catalyst system of 3a/Mg(n)Bu(2) 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 (n)BuLi displayed an extremely high activity (1.1 x 10(6) g mol(Sc)(-1) h(-1)) and afforded syndiotactic PMMA (rr = 75%) at low polymerization temperature (-20 degrees C) in THF.