Six members of a new family of {ONSO}-type ligands-the tetradentate-diainionic imine-thiobis(phenolate) ligands-were prepared by a two-step synthesis: Their [{ONSO}Zr(O(t)Bu)(2)] complexes formed as single diastereomers with different degrees of fluxionality that depended on the substitution pattern of the two phenolate rings. The complexes were active in polymerization of L-lactide and rac-lactide. The tacticity of the poly(lactic acid) prepared from rac-lactide changed gradually from heterotactically inclined through atactic to isotactically inclined. The most flexible complex and the most rigid complex gave the highest degrees of heterotacticity and isotacticity, respectively.