Redox-inactive metals are found in biological and heterogeneous water oxidation catalysts, but, at present, their roles in catalysis are not well understood. Here, we report a series of high-oxidation-state tetranuclear-dioxido clusters comprising three manganese centres and a redox-inactive metal (M). Crystallographic studies show an unprecedented Mn3M(µ4-O)(µ2-O) core that remains intact on changing M or the manganese oxidation state. Electrochemical studies reveal that the reduction potentials span a window of 700 mV and are dependent on the Lewis acidity of the second metal. With the pKa of the redox-inactive metal-aqua complex as a measure of Lewis acidity, these compounds demonstrate a linear dependence between reduction potential and acidity with a slope of ∼100 mV per pKa unit. The Sr(2+) and Ca(2+) compounds show similar potentials, an observation that correlates with the behaviour of the oxygen-evolving complex of photosystem II, which is active only if one of these two metals is present.