We have performed a comprehensive study of the electronic structure and magnetic properties of structurally characterized models for diiron-oxo proteins. Results from Kohn-Sham density functional theory show that two complexes, with formula Fe-2(mu-O)(mu-O2CCH3)(2)(HBpz(3))(2) and [Fe-2(mu-OH)(mu-O2CCH3)(2)(HBpz(3))(2)](+), are strongly and weakly antiferromagnetically coupled, respectively, in agreement with experiment. The physical origin of the stronger and weaker exchange typically measured for oxo- and hydroxo-bridged diiron complexes, respectively, has been elucidated. The main superexchange pathways giving rise to molecular antiferromagnetism in both complexes have been identified. The dominant pathway in the oxo-bridged complex, Fe1(d(xz)):mu-O(p(x)):Fe2(d(xz)), was formed by pi interactions whereas that of the hydroxo-bridged, Fe1(d(z)(2)):mu-O-H(p(parallel to)):Fe2(d(z)(2)), was formed by sigma interactions. We also found a pathway mediated by the bridging acetates, Fe1(d(x)(2)-y(2)):bis(mu-acetato):Fe2(d(x)(2)-y(2)), which induces weak antiferromagnetism in the oxo-bridged complex but is significantly more important in the hydroxo-bridged complex. The antiferromagnetic exchange constants that parameterize the Heisenberg Hamiltonian H=JS(1).S-2 have been predicted for both, strongly and weakly, coupled complexes. Overall, the signs, trends, and magnitudes of the theoretical values (J(mu-O)(calc)=+152.7 cm(-1), J(mu-OH)(calc)=+23.3 cm(-1)) were in excellent agreement with experiment. The geometries of the complete molecular structures have been optimized in C-2v symmetry and used to calculate molecular properties such as atomic charges and spin densities. The electronic configurations (Fe:4s(0.29)3d(5.93),mu-O:2s(1.92)2p(4.99);Fe:4s(0.30)3d(5.82),mu-O-H:2s(1.82)2p(5.25),H:1s(0.51)) of the respective binuclear cores revealed relatively high occupancies for the nominally ferric ions, thus reflecting a donating character of their immediate N3O3 coordination. In addition, the diiron-oxo protein hemerythrin has been discussed. Theoretical and structural considerations indicated that the oxo-bridged diferric complex considered herein models extremely well the antiferromagnetic behavior of azidomet- and azidometmyo-hemerythrin. Finally, the magnetic behavior of closely related oxo-bridged diferric and hydroxo-bridged diferrous complexes containing Me(3)TACN capping ligands has been explained in light of the results presented in this work.