The understanding of the structure-function relationship of the oxygen-evolving center(OEC), a Mn_4 Cacluster, in photosystem II is impeded mainly due to the complexity of the protein environment and lack of rational chemical models as a reference. In this study, two novel Mn_4-oxido complexes have been synthesized and characterized, in which the peripheral ligands of the [Mn_4~Ⅲ] core are provided by eight μ_2-carboxylate groups and two neutral terminal ligands(pyridine or isoquinoline). This type of peripheral ligation is very similar to the Mn_4Ca-oxide model complexes recently reported to mimic the OEC. The new Mn_4-oxide complex can catalyze the oxygen-evolving reaction in the presence of Bu~tOOH as an oxidant. The structure and redox properties comparison of the Mn_4-oxido and Mn_4Ca-oxido complexes provide important clues to understanding the functional role of Ca in the OEC in natural photosynthesis, and develop more efficient artificial catalysts for the water-splitting reaction in the future. The understanding of the structure-function relationship of the oxygen-evolving center (OEC), a Mn_4Cacluster, in photosystem II is impeded due due to the complexity of the protein environment and lack of rational chemical models as a reference. In this study, Two novel Mn_4-oxido complexes have been synthesized and characterized, in which the peripheral ligands of the [Mn_4 ~ III] core are provided by eight μ_2-carboxylate groups and two neutral terminal ligands (pyridine or isoquinoline). This type of peripheral ligation is very similar to the Mn_4Ca-oxide model complexes recently reported to mimic the OEC. The new Mn_4-oxide complex can catalyze the oxygen-evolving reaction in the presence of Bu ~ tOOH as an oxidant. The structure and redox properties comparison of the Mn_4- oxido and Mn_4Ca-oxido complexes provide important clues to understanding the functional role of Ca in the OEC in natural photosynthesis, and develop more efficient artificial catalysts for the water-splitting rea ction in the future.