We have synthesized a diiron(II) complex with a novel aqua-hydroxo bridging motif, [Fe-2(u-H2O)(u-OH)(TPA)(2)](OTf)(3) (1). This is a new member of the diiron diamond core family. The complex is stable in solution in nonpolar solvents as well as in the solid state. Two high-spin iron(II) sites are antiferromagnetically coupled (J = -9.6 cm(-1)). The drastic difference of ca. 1 V in the redox potential between complex 1 and its bis(hydroxo)-bridged analogue Fe-2(OH)(2)(TPA)(3+) is accompanied by only a moderate difference in the dioxygen reactivity. This observation is consistent with the inner-sphere mechanism of iron(II)-dioxygen association rather than the outer-sphere electron transfer.