The dinuclear complex [(susan){Fe-III(OH)(mu-O)FeOIII(OH)}](ClO4)(2) (Fe-2(OH)(2)(ClO4)(2); susan = 4,7-dimethyl-1,1, 10,10-tetra (2-pyridylmethyl)-1,4,7,10-tetraazadecane) with two unsupported terminal hydroxido ligands and for comparison the fluorido-substituted complex [(susan)-{(FeF)-F-III(mu-O)(FeF)-F-III}] (ClO4O2 (Fe2F2(ClO4)(2)) have been synthesized and characterized in the solid state as well in acetonitrile (CH3CN) and water (H2O) solutions. The Fe-OH bonds are strongly modulated by intermolecular hydrogen bonds (1.85 and 1.90 angstrom). UV-vis-near-IR (NIR) and Mossbauer spectroscopies prove that Fe2F22+ and Fe-2(OH)(2)(2+) retain their structural integrity in a CH3CN solution. The OH- ligand induces a weaker ligand field than the F- ligand because of stronger pi donation. This increased electron donation shifts the potential for the irreversible oxidation by 610 mV cathodically from 1.40 V in Fe2F22+ to 0.79 V versus Fc(+)/Fc in Fe-2(OH)(2)(2+). Protonation/deprotonation studies in CH3CN and aqueous solutions of Fe-2(OH)(2)(2+) provide two reversible acid-base equilibria. UV-vis-NIR, Mossbauer, and cryo electrospray ionization mass spectrometry experiments show conservation of the mono(mu-oxo) bridging motif, while the terminal OH- ligands are protonated to H2O. Titration experiments in aqueous solution at room temperature provide the pK(a) values as pK(1) = 4.9 and pK(2) = 6.8. Kinetic studies by temperature- and pressure-dependent O-17 NMR spectrometry revealed for the first time the water exchange parameters [k(ex)(298) = (3.9 +/- 0.2) X 10(5) s(-1), Delta H-double dagger = 39.6 +/- 0.2 kJ mol(-1) Delta S-double dagger = -5.1 +/- 1 J mol(-1) K-1, and Delta V-double dagger = +3.0 +/- 0.2 cm(3) mol(-1) and the underlying I-d mechanism for a {Fe-III(OH2)(mu-O)Fe-III(OH2)} core. The same studies suggest that in solution the monoprotonated {Fe-III(OH)(mu-O)Fe-III(OH2)} complex has mu-O and mu-O2H3 bridges between the two Fe centers.