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 (μ4-O)(μ2-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 p K a of the M-
aqua complex as a measure of Lewis acidity, these compounds display a linear …