The iron K-edge EXAFS spectra of methemerythrin azide (metHrN3), semimethemerythrin azide (semimetHrN3), and the B2subunit of ribonucleotide reductase from Escherichia coli (RRB2) have been analyzed and compared with those of several binuclear and tetranuclear model compounds. The synthetic complexes include compounds with hydroxo and oxo bridges and several with the triply bridged diiron core similar to that found in metHrN3. The analysis allows the determination of the presence of a short Fe-O distance among longer Fe-O(N) distances and the estimation of average bond lengths and the second-shell Fe-Fe distance with esd's of 0.015 and 0.02 A, respectively. Both metHrN3and RRB2exhibit short Fe-O (1.79 and 1.78 A, respectively) bonds and similar Fe-Fe (3,19 and 3.22 A, respectively) distances, suggesting that the metal sites of the two proteins share the same (M-oxo)bis(ju-carboxylato)diiron core. However, the average bond length for the remaining ligands in the first coordination sphere of RRB2is 0.07 A shorter than that of metHrN3, This suggests that at least one histidine per iron in metHrN3is replaced by an oxyanion ligand in RRB2, since Fe-O bonds are significantly shorter than Fe-N bonds in high-spin ferric complexes. The analysis for semimetHrN3indicates that the short Fe-O bonds found in metHrN3have significantly lengthened in semimetHrN3. The Fe-Fe distance increases to 3.46 A, while the average bond length for the other ligands increases by only 0.01 A. These results are interpreted in terms of a localized mixed valent Fe(III)-Fe(II) complex bridged by hydroxide and two carboxylates, consistent with previously reported NMR and Raman data.