A series of tetranuclear oxo/hydroxo clusters comprised of three Fe centers and a redox-inactive metal (M) of various charge is reported. Crystallographic studies show an unprecedented Fe3M(mu(4)-O)(mu(2)-OH) core that remains intact upon changing M or the oxidation state of iron. Electrochemical studies reveal that the reduction potentials (E-1/2) span a window of 500 mV and depend upon the Lewis acidity of M. Using the pK(a) of the M-aqua complex as a measure of Lewis acidity, these compounds display a linear dependence between E-1/2 and acidity, with a slope of similar to 70 mV per pK(a) unit. The current study of [Fe3MO(OH)] and previous ones of [Mn3MOn] (n = 2,4) moieties support the generality of the above relationship between the reduction potentials of heterometallic oxido clusters and the Lewis acidity of incorporated cations, as applied to clusters of different redox-active metals.