Photosynthetic oxygen-evolving center (OEC), the "engine of life", is a unique Mn 4 CaO 5 cluster catalyzing the water oxidation. The role of redox-inactive component Ca 2+ , which can only be functionally replaced by Sr 2+ in a biological environment, has been under debate for a long time. Recently, its modulating effect on the redox potential of native OEC and artificial structural OEC model complex has received great attention, and linear relationship between the potential and the Lewis acidity of the redox-inactive metal has been proposed for the MMn 3 O 4 model complex. In this work, the modulating effect has been studied in detail using the Mn 4 CaO 4 model complex, which is the closest structural model to OEC to date and has a similar redox potential at the S 1 −S 2 transition. We found the redox-inactive metal only has a weak modulating effect on the potential, which is comparable in strength to that of the ligand environments. Meanwhile, the net charge of the complex, which could be changed along with the redox-inactive metal, has a high impact on the potential and can be unified by protonation, deprotonation, or ligand modification. Although the modulating effect of the redox-inactive metal is not very strong, the linear relationship between the potential and the Lewis acidity is still valid for Mn 4 MO 4 complexes. Our results of strong modulating effects for net charge and weak modulating effects for redox-inactive metal fit with the previous experimental observations on Mn 4 MO 4 (M = Ca 2+ , Y 3+ , and Gd 3+ ) model complexes, and suggest that Ca 2+ can be structurally and electrochemically replaced with other metal cations, together with proper ligand modifications.

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