The syntheses, structures, and lactide polymerization initiation are reported for a series of novel [{N,N'-1,3-bis(P,P'-di-isopropyloxophosphinic)-2,2-dimethylpropylenediamido}(amido)yttrium] complexes and contrasted with [{N,N'-1,3-bis(P,P'-di-isopropylthiophosphinic)-2,2-dimethylamido}{bis(trimethylsilyl)-amido}yttrium]. The syntheses of the novel [{N,N'-1,3-bis(P,P'-di-isopropyloxo/thiophosphinic)-2,2-dimethylpropylenediamido}(amido)yttrium] complexes were achieved in excellent yields by reaction of the N,N'-1,3-bis(P,P'-di-isopropyloxo/thiophosphinic)-2,2-dimethylpropylenediamine ligands with [Y(NR(2))(3)center dot xTHF] (R = SiMe(3), SiMe(2)H, i-Pr, x = 0, 2). The new complexes were characterized by NMR and IR spectroscopies, elemental analyses, and for R = SiMe(2)H X-ray crystallography. The solution and solid-state structures were compared using 21) NMR techniques and in particular using pulsed gradient spin echo spectroscopy (PGSE) to derive the solution hydrodynamic radius and compare it to that calculated from the X-ray crystal structure: the [{N,N'-bis(P,P'-di-isopropyloxophosphinic)-2,2-dimethylpropylenediamido}(amido)yttrium] complexes were all dimeric in both the solid state and solution. They showed unusual structures where each ligand has one oxophosphinic group bonded to a single yttrium center and one oxophosphinic group bonded to two yttrium centers. The new complexes were tested as lactide polymerization initiators; they all showed very high activities. The degree of polymerization control exerted by the novel initiators was related to the size of the initiating amide group and the structure of the complex.