A series of constrained-geometry-con?guration (CGC) rare-earth metal complexes bearing the ligand ?uorenylmethylene-tetrahydrofuranyl (η⁵ -FluCH₂-THF)Ln(CH₂SiMe₃)₂(THF) (Ln = Y, 1; Lu, 2) have been synthesized by the alkane elimination reaction between the FluCH₂-THF ligand and the rare-earth metal trialkyls and fully characterized by nuclear magnetic resonance and X-ray di?raction analyses. Complexes 1 and 2 are monomeric THF solvates, in which the CGC ligand adopts the η⁵ /κ¹ bonding mode with the central metals via coordination of ?ve carbon atoms from the ?uorenyl fragment and the tetrahydrofuranyl oxygen atom. Upon activation with AlⁱBu₃ and [Ph₃C][B(C₆F₅)₄], complex 1 showed high activity toward conjugated diene polymerization, to a?ord up to 98.5% cis-1,4-polybutadiene and 69.0% 3,4-unit-enriched polyisoprene. The reason behind this was elucidated.