We have synthesized the first complexes with bis(mu-oxo)diiron(III) and (mu-oxo)(mu-hydroxo)diiron-(III) cores (1 and 2, L = TPA (a), 5-Et-3-TPA (b), 6-Me-3-TPA (c), 4,6-Me-6-TPA (d), BQPA (e), BPEEN (f), and BPMEN (g)) and found them to have novel structural properties. In particular, the presence of two single-atom bridges in these complexes constrains the Fe-Fe distances to 2.7 - 3.0 Angstrom and the Fe-mu-O-Fe angles to 100 degrees dr smaller. The significantly acute Fe-O-Fe angles (e.g, 92.5(2)degrees for 1c and 100.2(2)degrees for 2f) enforced by the Fe2O2(H) core endow these complexes with UV-vis, Raman, and magnetic properties quite distinct from those of other (mu-oxo)diiron(III) complexes. Complex Ic exhibits visible absorption bands at 470 (epsilon = 560 M-1 cm(-1)) and 760 nm (epsilon approximate to 80 M-1 cm(-1)), while complexes 2 show features at ca. 550 (epsilon approximate to 800 M-1 cm(-1)) and ca. 800 nm (epsilon approximate to 70 M-1 cm(-1)), all of which are red shifted compared to those of other (mu-oxo)-diiron(III) complexes. These complexes also exhibit distinct nu(Fe-O-Fe) vibrations at ca. 600 and ca. 670 cm(-1) assigned to the nu(sym) and the nu(asym) of the Fe-O-Fe units, respectively. The relative intensities of the nu(sym) and nu(asym) bands are affected by the symmetry of the Fe-O-Fe units; an unsymmetric core enhances the intensity of the nu(asym). Complexes 2 exhibit another band at ca. 500 cm(-1), which is assigned to the Fe-(OH)-Fe stretching mode due to its sensitivity to both (H2O)-O-18 and (H2O)-H-2. Magnetic susceptibility studies reveal J = 54 cm(-1) for Ic and ca. 110 cm(-1) for 2 (H = JS(1).S-2), values smaller than those for the antiferromagnetic interactions found in (mu-oxo)diiron(III) complexes. This weakening arises from the longer Fe-mu-O bonds and the smaller Fe-mu-O-Fe angles in the Fe2O2(H) diamond core structure. These spectroscopic signatures can thus serve as useful tools to ascertain the presence of such core structures in metalloenzyme active sites. These two core structures, Fe-2(mu-O)(2) (1) and Fe-2(mu-O)(mu-OH) (2), can also be interconverted by protonation equilibria with pK(a)'s of 16-18 in CH3CN. Furthermore, the Fe-2(mu-O)(2) core (1) isomerizes to the Fe-3(mu(2)-O)(3) core (7), while the Fe-2(mu-O)(mu-OH) core (2) exhibits aquation equilibria to the Fe-2(mu-O)(mu-H3O2) core (5), except for L = 6-Me3-TPA and 4,6-Me-6-TPA. It is clear from these studies that electronic and steric properties of the ligands significantly affect the various equilibria, demonstrating a rich chemistry involving water-derived ligands alone.