In view of the. biological and commercial interest in models for Oxalate Decarboxylases (OxDC) and Oxalate Oxidases (OxOx), we have synthesized and characterized three new Mn-II complexes (1-3) employing N3O-donor aminocarboxylate ligands (TCMA, 1,4,7-triazacyclononane-N-acetic acid; K(i)Pr(2)TCMA, potassium 1,4-diisopropyl-1,4,7-triazacyclononane-N-acetate; and KBPZG, potassium N,N-bis(3,5-dimethylpyrazolyl methyl)glycinate). These complexes were characterized by several techniques including X-ray crystallographic analysis, X-band electron paramagnetic resonance (EPR), electrospray ionization mass spectrometry (ESI-MS), and cyclic voltammetry. The crystal structures of 1 and 3 revealed that both form infinite polymeric chains of Mn-II complexes linked by the pendant carboxylate arms of the TCMA(-) and the BPZG(-) ligands in a syn-antipattern. Complex 2 crystallizes as a mononuclear Mn-II cation, six-coordinate in a distorted octahedral geometry. Although complexes 1 and 3 crystallize as polymeric chains, all compounds present the same N3O-donor set atoms around the metal center as observed in the crystallographically characterized OxDC and OxOx. Moreover, complex 2 also contains two water molecules coordinated to the Mn center as observed in the active site of OxDC and OxOx. ESI-MS spectrometry, combined with EPR, were useful techniques to establish that complexes 1-3 are present as mononuclear Mn-II species in solution. Finally, complexes 1-3 are able to model the resting state active sites, with special attention focused on complex 2 which provides the first exact first coordination sphere ligand structural model for the resting states of both OxDC and OxOx.