Using broken-symmetry density functional theory and spin-projection methods, we have examined the electronic structure and properties of a large mixed-valent Fe(III)-Fe(IV) diiron system that displays two bidentate carboxylates and a single mu-oxo moiety as bridging ligands. Two carboxylates and a single oxygen species have long been implicated as core elements of the elusive intermediate X in ribonucleotide reductase. Spectroscopic studies of X have also identified the presence of an additional terminal or bridging oxygen-based ligand. Introduction of a second oxygen and protonated variants thereof in the core of our structural model is favored as a bridging hydroxide based on the lowest energy structure. Mössbauer measurements indicate clearly that the two iron sites of X are distinct and that there is significant electron delocalization onto the oxygen-based ligands. For several examined spin states of our model cluster, Mössbauer parameters from density functional calculations are neither able to differentiate between the iron sites nor reproduce the strong spin delocalization onto the oxygen-based ligands observed experimentally. The combined comparison of the calculated geometries, spin states, spin densities, and Mössbauer properties for our model clusters with available experimental data for X implies that intermediate X is significantly different from the diiron structural models examined herein.