In quest of new, single-site catalysts for cyclic ester polymerizations, a series of mononuclear yttrium(III) complexes of N,N'-bis(trimethylsilyl)benzamidinate ([L-TMS](-)) and hindered N,N'-bis-(2,6-dialkylaryl)toluamidinates ([L-Et] aryl = Et2C6H3, and [L-iPr](-), aryl = (Pr2C6H3)-Pr-i) were synthesized and characterized by X-ray diffraction: (L2Y)-Y-TMS(mu -Cl)(2)Li(TMEDA) (1), (L2Y)-Y-TMS((OC6H2Bu2Me)-Bu-t) (2), (L2Y)-Y-TMS(OC6H3Me2)(2)Li(THF)(4) (3), (L2Y)-Y-TMS(mu -(OBu)-Bu-t)(2)Li(THF) (4), (LY)-Y-iPr[N(SiMe2H)(2)](2)(THF) (5), (L2Y)-Y-Et(THF)(Cl)(mu -Cl)Li(THF)(3) (6), and (L2Y)-Y-Et[N(SiMe2H)(2)] (7). Coordination numbers ranging from five to seven were observed, and they appeared to be controlled by the steric bulk of the supporting amidinate and alkoxide, phenoxide, or amide coligands. Complexes 2-5 and 7 are active catalysts for the polymerization of D,L-lactide (e.g., with 2 and added benzyl alcohol, 1000 equiv of D,L-lactide were polymerized at room temperature in less than 1 h, with polydispersities less than 1.5). The neutral complexes 2, 5, and 7 were more effective than the anionic complexes 3 and 4. In addition, the presence of the more hindered amidinate ligands [L-Et] and [L-iPr](-) On yttrium-amides slowed the polymerizations (7 < 5 < Y[N(SiMe2H)(2)](3)). (C) 2000 John Wiley & Sons, Inc.