A series of amidinate-based N,O-chelated magnesium complexes [(L1)2(THF)2Mg] (1), [(L2)2(THF)2Mg] (2), [(L3)2(THF)2Mg] (3), and [(L4)2Mg] (4) were prepared by treating N-benzoyl-N′-arylbenzamidines (L1-4H) with 0.5 equiv of di-n-butylmagnesium in THF. Analogous CH3CN-coordinated complexes [(L1)2(CH3CN)2Mg] (5) and [(L3)2(CH3CN)2Mg] (6) were prepared in a similar way using CH3CN as solvent. All of the compounds were characterized by 1H/13C NMR spectroscopy, and the molecular structures of 1, 2, and 4-6 were further confirmed by single-crystal X-ray diffraction studies. Complexes 1, 2, 5, and 6 displayed good catalytic activity toward the ring-opening polymerization (ROP) of ϵ-caprolactone. In addition, 1, 5, and 6 were also found to be excellent catalysts for making cyclic carbonates from CO2 and epoxides in the presence of a cocatalyst, n-Bu4NBr. © 2017 American Chemical Society.