The reaction of LnCl3(THF)n (Ln = La, n = 4; Ln = Y, Sm, Nd, n = 3) with 3 equiv of allylMgCl in THF/1,4-dioxane followed by crystallization in 1,4-dioxane/toluene proceeds in very high yields to give the tris(η3-allyl) complexes [La(η3-C3H5)3(κ1-dioxane)]2(μ-dioxane) and [Ln(η3-C3H5)3(μ-dioxane)]∞ (Ln = Y, 1; Sm, 2; Nd). Improved one-pot syntheses for the tris(η3-allyl) lanthanum and neodymium compounds are reported. The crystal structures of 1 and 2 show that the compounds consist of distorted square-pyramidal lanthanide centers in linear polymer chains. The reaction of [La(η3-C3H5)3(κ1-dioxane)]2(μ-dioxane) and [Ln(η3-C3H5)3(μ-dioxane)]∞ with the diketimine ligand 2-(2,6-diisopropylphenyl)amino-4-(2,6-diisopropylphenyl)imino-2-pentene (BDI-H) in THF at 60 °C generates cleanly the β-diketiminato complexes Ln(η3-C3H5)2{κ2-HC(MeCNAr)2} (Ar = 2,6-C6H3iPr2; Ln = La, 3; Y, 4; Sm, 5; Nd, 6) with propene elimination. The crystal structure of 5 shows a distorted tetrahedral SmN2(allyl)2 core, with the samarium atom positioned 1.438(4) Å out of the C3N2 ligand plane. All complexes are highly effective single-component catalysts for the ring-opening polymerization of ε-caprolactone and rac-lactide. Polymer end group analysis shows that the polymerization process is initiated by allyl transfer to monomer.