The reaction of the mixed-metal lanthanide/magnesium allyl complexes [Ln(η3-C3H5)3(μ-C4H8O2)·Mg(η1-C3H5)2(μ-C4H8O2)1.5]n (Ln = La, 1; Y, 2) with the diketimine 2-(2,6-diisopropylphenyl)amino-4-(2,6-diisopropylphenyl)imino-2-pentene (BDI-H) in THF at 60 °C leads to the isolation of the magnesium β-diketiminato complex Mg{HC(MeCNC6H3Pri2-2,6)2}(η1-C3H5)(THF) (3). The same product was obtained in the reaction of BDI-H with the ionic allyl lanthanate compounds [Mg(THF)6][Nd(η3-C3H5)4]2·2THF (4) and [Mg(THF)6][Sm(η3-C3H5)4]2·2THF (5). No lanthanide-containing product could be isolated. The X-ray structure of 3 shows that the complex is a monomeric neutral species with the magnesium center in a distorted tetrahedral arrangement. Complex 3 was found to hydrolyze after repeated crystallization attempts at −26 °C to give the hydroxy complex [Mg{HC(MeCNC6H3Pri2-2,6)2}(μ-OH)(THF)]2·4THF (6). The X-ray structure of 6 consists of a hydroxide-bridged dimer with the magnesium center in a distorted square pyramid. Lanthanide BDI compounds were, however, obtained by reacting LnCl3(THF)x (Ln = La, x = 4; Ln = Y, x = 3) with the lithium salt [Li(BDI)(THF)]2 to give the dimeric complexes [LnCl2{HC(MeCNC6H3Pri2-2,6)2}]2(THF)·2toluene (Ln = La, 7; Y, 8). According to its X-ray structure, 8 possesses distorted octahedral yttrium centers, one terminal chloride, and three bridging chloride ligands. The reaction of 7 or 8 using 4 equiv of allylMgCl did not give the expected mixed-ligand allyl lanthanide complex but led again to the magnesium complex 3 via BDI ligand exchange. Complexes 1−5 are highly effective single-component catalysts for the ring-opening polymerization of ε-caprolactone. Complex 3 also polymerizes rac-lactide efficiently and under mild conditions but with low stereospecificity. ε-Caprolactone is polymerized in minutes to high molecular weight materials by an allyl end-group transfer mechanism. The catalysts are active over a wide temperature range.