The complex [L(H2O)Fe(mu-O)Fe(OH)L](ClO4)(3). H2O, where L=N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)ethane-1,2-diamine, was synthesized. It crystallizes in the orthorhombic space group P2(1)2(1)2(1) With a = 13.283(5) Angstrom, b = 16.050(9) Angstrom, c = 20.050(9) Angstrom, V = 4476(6) Angstrom(3), and Z = 4. It presents the hydrogen-bonded [(H2O)Fe(mu-O)Fe(OH)](3+) core unit characterized by an Fe-O-Fe angle of 137.5(2)degrees and an Fe-Fe distance of 3.396(1) Angstrom. The measurement of the magnetic susceptibility as a function of the temperature indicated an antiferromagnetic coupling between the two high-spin Fe(III) ions J = -184 cm(-1) (H = -JS(1). S-2). In the solid state the symmetric stretching vibration was observed at 438 cm(-1). Upon dissolution in dry acetonitrile,this vibration was no longer detected and an intense band was observed at 600 cm(-1). This frequency can be correlated with an Fe-O-Fe angle of 111 degrees. This value suggests that the species which exists in these conditions is the protonated diamond core [Fe(mu-O)(mu-OH)Fe](3+), analogous to that identified by Zang et al. (J. Am. Chem. Sec. 1994, 116, 3653) and by Hazell et al. (J. Chem. Sec.; Dalton Trans. 1995, 707). Upon addition of water, the original aquated species is observed in equilibrium with the protonated diamond unit. By:analysis of the changes in UV-vis spectra as a function of the amount of water added, the equilibrium constant of the formation of the aquated species was found to be 5.4 M-1.