Complexes containing a phosphate- or acetate-bridged (mu-hydroxo)diiron(III) core, such as may exist in the metalloproteins uteroferrin, beef spleen purple acid phosphatase, or the hydroxylase component of methane monooxygenase, have been prepared. A general procedure is presented for synthesizing hydroxo-bridged complexes by protonation of their oxo-bridged analogues in ether solution. As a result of protonating the bridging oxygen atom, the dimetallic core in the hydroxo-bridged complexes is expanded (Fe-OH = 1.95-1.97 angstrom, Fe...Fe = 3.44-3.59 angstrom, angle Fe-OH-Fe = 123-131-degrees) relative to that in the corresponding mu-oxo complexes (Fe-O = 1.78-1.81 angstrom, Fe...Fe = 3.10-3.33 angstrom, angle Fe-O-Fe = 120-130-degrees), with a concomitant decrease in the magnitude of the antiferromagnetic spin exchange coupling constants (J almost-equal-to -10 to -20 cm-1) and correspondingly larger paramagnetic shifts (-20 to -70 ppm) in the H-1 NMR spectra. Electronic transitions differ considerably from those of oxo-bridged complexes, and Mossbauer quadrupolar coupling parameters (0.25-0.40 mm/s) are considerably smaller in the hydroxo-bridged analogues. A new diiron(III) complex containing three bidentate bridging groups, [Fe2{O2P(OC6H5)2}3(HBpz3)2]-(BF4), was also prepared and characterized spectroscopically, magnetically, and structurally. In this weakly antiferromagnetically coupled compound (J = -0.8 cm-1), the Fe...Fe distance is 4.677(1) angstrom. Unit cell data : [Fe2(OH)(O2CCH3)2(HBPZ3)2](ClO4).CH2Cl2, space group C5(2h)-P2(1)/n, a = 11.757(2) angstrom, b = 19.925(4) angstrom, c = 15.580(2) angstrom, beta = 92.03(1)degrees, V = 3647(2) angstrom3, Z = 4, T = -15-degrees-C; [Fe2(OH){O2P(OC6H5)2}2(HBPZ3)2](BF4).2CH2-Cl2.0.5(CH3CH2)2O.H2O, space group C6(2h)-C2/c, a = 21.156(4) angstrom, b = 20.518(2) angstrom, c = 27.157(4) angstrom, beta = 104.268(7)degrees, V = 11 425(3) angstrom3, Z = 8, T = -38-degrees-C; [Fe2(OH){O2P(C6H5)2}2(HBpz3)2](BF4).2.5(CH3)2CO, space group C3(2h)-C2/m, a = 16.772(2) angstrom, b = 33.585(4) angstrom, c = 10.902(1) angstrom, beta = 105.206(8)degrees, V = 5926(2) angstrom3, Z = 4, T = -54-degrees-C; [Fe2{O2P(OC6H5)2}3(HBPZ3)2](BF4).CH2Cl2, space group C1i-P1BAR, a = 12.346(2) angstrom, b = 15.084(2) angstrom, c = 17.633(2) angstrom, alpha = 97.50(1)degrees, beta = 99.02(1)degrees, gamma = 95.76(1)degrees, V = 3191(2) angstrom3, Z = 2, T = -54-degrees-C.