The synthesis, molecular structures, and spectroscopic properties of a series of valence-delocalized diiron(II,III) complexes are described. One-electron oxidation of diiron(II) tetracarboxylate complexes afforded the compounds [Fe-2(mu-O2CArTol)(4)L-2]X, where L = 4-(BuC5H4N)-Bu-t (1b), C5H5N (2b), and THF (3b); X = PF6- (1b and 3b) and OTf- (2b). In 1b-3b, four mu-1,3 carboxylate ligands span relatively short Fe...Fe distances of 2.6633(11)-2.713(3) Angstrom. Intense (epsilon = 2700-3200 M-1 cm(-1)) intervalence charge transfer bands were observed at 620-670 nm. EPR spectroscopy confirmed the S = (9)/(2) ground spin state of 1b-3b, the valence-delocalized nature of which was probed by X-ray absorption spectroscopy. The electron delocalization between paramagnetic metal centers is described by double exchange, which, for the first time, is observed in diiron clusters having no single-atom bridging ligand(s).