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Stoichiometric reactions of YCl3(THF)3withpotassium 2-((arylimino)methyl)quinolin-8-olates or 2-(1-(arylimino)ethyl)quinolin-8-olatesin THF solution gave the mononuclear LYCl2(DMSO)2complexes 1–5in the presence ofDMSO and a representative dinuclear complex 6in theabsence of DMSO. All yttrium complexes were fully characterized byNMR measurements and elemental analysis, and the crystal structuresof complexes 1and 4–6were determined by single-crystal X-ray diffraction. The structuresindicate coordination number seven around the yttrium center and pentagonalbipyramidal geometries. The complexes all feature diapical YCl2moieties and one tridentate organic ligand in the equatorialplane. Upon reaction of the yttrium precatalysts 1–6with LiCH2Si(CH3)3aloneor with LiCH2Si(CH3)3together withBnOH, the ring-opening polymerization (ROP) of ε-caprolactone(ε-CL) occurred with high efficiency. Depending on conditions,the ROP of ε-CL produced polycaprolactone with narrow moleculardistribution and in a living manner. Theoretical studies of the chlorine/CH2SiMe3and Me3SiCH2/BnO ligandexchange reactions suggest that the replacement of the apical ligandscan proceed without significantly affecting the equatorial ligands.These results suggest that one of the apical Y–CH2SiMe3bonds within the LY(CH2SiMe3)2intermediate catalyzes the polymerization in the BnOH-freeprocess. Most polymers generated by BnOH-assisted catalysis possess Mnvalues that are similar to Mn,calvalues based on Y–OBn, suggesting that oneapical Y–OBn bond of the diapical LY(OBn)(CH2SiMe3) intermediate catalyzes most or all of the ring polymerizationof ε-CL. [ABSTRACT FROM AUTHOR]