The syntheses of a series of bis(thiophosphinic amido) yttrium complexes are reported by the reaction of bis(thiophosphinic amine) ligands with [Y(N(SiMe3)(2))(3)]. The complexation reactions were monitored using NMR spectroscopy and one complex was characterized by single crystal X-ray crystallography. Two substitution sites on the ligand were varied, the phosphorus substiments were phenyl, isopropyl and ethoxy groups and the diamine backbones were ethylene, trans-cyclohexylene and 2,2-dimethylpropylene groups. All the new complexes were tested as initiators for the ring opening polymerization of rac-lactide. They exhibited good polymerization control, shown by the linear fits to plots of number-averaged molecular weight (M.) versus the percentage conversion, the close agreement between the theoretical and observed degree of polymerization (DP) and the moderate polydispersity index (PDI) values. They also showed very high polymerization rates (k(app) = 2.2 x 10(-4) to 1.1 x 10(-2) s(-1) at [lactide](0) = 1 M, [initiator](0) = 5 mM). The phosphorus substituents had the greatest influence over the rate, with the order of decreasing rate being isopropyl > phenyl > ethoxy. The complexes with ethoxy phosphorus substiments exerted good stereocontrol; when rac-lactide was polymerized it formed predominantly heterotactic polylactide (P-S = 0.69-0.79).