Two divalent ytterbium (Yb-II) complexes, 1 and 2, supported by new [ONOO]-type amine bis(phenolate) ligands L-a,L-b were synthesized in good yield by an amine elimination reaction of Yb-II(N(SiMe3)(2))(2)(TMEDA) (TMEDA = tetramethylethylenediamine) with one equivalent of the ligand precursor. X-ray structural determination showed complexes 1 and 2 both have a monomeric structure. Each adopts distorted octahedral coordination geometry around the six-coordinate Yb-II ion. Two new trivalent ytterbium (Yb-III) aryloxide complexes, 4 and 5, bearing [ONOO]-type amine bis(phenolate) ligands L-c and L-d were prepared by double-protonation reaction of Yb(C5H5)(3)THF with one equivalent of the ligand precursor, then one equivalent of phenol. Complex 4 has a symmetric dimeric structure with a Yb2O2 core bridging through the oxygen atoms of the OC6H4-4-CH3 groups. Complex 5 is a THF-solvated monomer. Each six-coordinated Yb-III ion in both complexes adopts a distorted octahedron. All the complexes synthesized together with the known Yb-II complex 3 were evaluated in the ring-opening polymerization (ROP) of rac-lactide (rac-LA). Complexes 1 and 4 were found to be extremely active for controlled ROP of rac-LA, as judged by narrow molar mass distributions (M-w/M-n: 1.07-1.16 for complex 1 and 1.07-1.10 for complex 4) and experimental molar mass M-n,M-exp values in good agreement with theoretic M-n,M-calcd values calculated on a single PLA chain produced per metal center of initiator. Complex 5 is less controlled. Complexes 1, 4, and 5 exhibited the same high stereoselectivity to give heterotactic polylactide with a P-r (probability of racemic enchainment of monomer units) ranging from 0.97 to 0.99. Dramatic decreases in activity and stereoselectivity were found for complexes 2 (P-r = 0.82) and 3 (P-r = 0.67), indicating the framework of L in the unit of [YbL] is crucial for determining activity and stereoselectivity of either Yb-II or Yb-III complexes.