Two novel bidentate and tridentate anilido-aldimine ligands, [o-C6H4(NHAr)—HC=NCH2C6H5] (AABn-H) and [o-C6H4(NHAr)—HC=NCH2CH2OCH3] (AANO-H) (Ar = 2,6-iPr2C6H3) have been prepared by treating 2-(2,6-diisopropyl-phenylamino)-benzaldehyde (C) in methanol with one molar equivalent of RNH2 (AABn-H: R = CH2C6H5; AANO-H: R = CH2CH2OCH3). To achieve single site zinc (Zn) complexes supported by the bidentate anilido-aldimine ligand, AABn-H, has been synthesized and structurally characterized. Reaction of AABn-H with one molar equivalent of ZnEt2 produces the three-coordinated monomeric complex [(AABn)ZnEt] (1), followed by the addition of 4-N,N-dimethylaminopyridine (DMAP) gives the four-coordinated monomeric complex [(AABn)Zn(DMAP)Et] (2). The four-coordinated monomeric complex [(AABn)2Zn] (3) was prepared by the reaction of half an equivalent of ZnEt2 with one molar equivalent AABn-H. Experimental results show that complexes (1)-(3) are efficient catalysts for ring-opening polymerization (ROP) of -caprolactone (-CL) in the presence of 9-anthracenemethanol (9-AnOH); among them, complex (1) catalyzes the polymerization of -CL in a controlled fashion yielding polymers with a narrow polydispersity indexes (PDIs). Additionally, complex (1) is also an efficient catalyst for ROP of -butyrolactone (-BL) in a controlled manner yielding polymers with expected molecular weights and narrow PDIs. Single site Mg and Zn complexes incorporated with the other anilido-aldimine ligand, AANO, have been successfully synthesized and structurally well-characterized. Reaction of AANO-H with one molar equivalent or half an equivalent amount of MgnBu2 or ZnEt2 affords the three-coordinated complex [(AANO)ZnEt] (4), four-coordinated complex [(AANO)2Zn] (6), [(AANO)MgnBu] (7) and six-coordinated complex[(AANO)2Mg] (8), respectively. The four-coordinated complex [(AANO)Zn(μ-OBn)]2 (5) derives from complex (4) by the addition of one molar equivalent benzyl alcohol. Experimental results show that complexes (5)-(8) efficiently catalyze the ROP of -CL/-BL in a controlled fashion yielding polymers with very narrow PDIs in a wide range of monomer-to-initiator ratios. In polymerizations, the activities of complexes (7) and (8) are highest among these complexes. It seems that the higher Lewis acidity and better oxophilic nature of Mg2+ metal provide a better combination of monomer activation and alkoxide nucleophilicity.