The reaction of mononuclear [LFeIII] where L represents the trianionic ligand 1,4,7-tris(4-tert-butyl-2-mercaptobenzyl)-1,4,7-triazacyclononane with NiCl2. 6H(2)O and subsequent oxidations with [Ni-III(tacn)(2)](ClO4)(3) (tacn = 1,4,7-triazacyclononane) and PbO2/methanesulfonic acid produced an isostructural series of complexes [LFeNiFeL](n+) (n = 2 (1), n = 3 (2), n = 4 (3)), which were isolated as PF6- (1, 3) or ClO4- salts (2). The molecular structures were established by X-ray crystallography for [LFeNiFeL](ClO4)(2). 5CH(3)CN (1*), C88H123Cl2Fe2N11NiO8S6, and [LFeNiFeL](ClO4)(3). 8acetone (2*), C102H156Cl3Fe2N6NiO20S6. Both compounds crystallize in the triclinic space group P (1) over bar with a = 13.065(2) Angstrom (13.155(2) Angstrom), b = 13.626(3) Angstrom (13.747(3) Angstrom), c = 14.043(3) Angstrom (16.237(3) Angstrom), alpha = 114.47(3)degrees (114.20(2)degrees), beta = 97.67(3)degrees (96.57(2)degrees), gamma = 90.34(3)degrees (98.86(2)degrees), Z = 1(1) (values in parentheses refer to 2*). The cations in 1, 2, and 3 have been determined to be isostructural by Fe and Ni K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. All compounds contain linear trinuclear cations (face-sharing octahedral) with an N3Fe(mu-SR)(3)Ni(mu-SR)(3)FeN3 core structure. The electronic structures of 1, 2, and 3 have been studied by Fe and Ni K-edge X-ray absorption near edge (XANES), UV-vis, EPR, and Mossbauer spectroscopy as well as by temperature-dependent magnetic susceptibility measurements. Complexes 1 and 3 possess an S-t = 0 whereas 2 has an S-t = 1/2 ground state. It is shown that the electronic structures cannot be described by using localized valences (oxidation states). Delocalized models invoking the double-exchange mechanism are appropriate; i.e., spin-dependent delocalization via a double-exchange mechanism yields the correct ground state in each case, 1, 2, and 3 represent the first examples where double exchange stabilizes a ground state of minimum spin multiplicity.