The 18-electron complexes [M-II(C5R5)(arene)](+) (M = Fe : R = H or Me, arene= C6H6-nMen (n = 0-6), C6H5NMe2; or C6Me5NH2; M = Ru : R = Me, arene = C6Me6) are oxidized to Mm complexes between 0.92 and 1.70 V vs [FeCp2] according to a single-electron process that is reversible in SO2 if at least one of the rings is permethylated. The dinuclear complex [Fe-2(II)(fulvalenyl)(C6Me6)][PF6](2) is oxidized in two one-electron reversible waves in SO2 separated by 0.38 V to the mixed-valence species trication and to the 34-electron dioxidized tetracation. Stoichiometric oxidation of the yellow complexes [(FeCp)-Cp-II*(arene)][EX6] (EX6 = PF6 or SbCl6) is achieved by using SbCl5 in CH(2)Ch(2) at 20 degrees C or ShF(5) in SO2 at -10 degrees C or by Br-2 + [Ag][SbF6] and gives the purple 17-electron complexes [(FeCp)-Cp-III*(arene)] [SbX6](2) (X = F or Cl) if arene = hexa-, penta-, and 1,2,4,5-tetramethylbenzene. No oxidation is observed for complexes of less methylated arene ligands, which shows that the oxidation power of SbX5 is limited to 1.0 V vs [FeCp2] for monocations. The complex [(FeCp)-Cp-III*(C6Me6)] [SbCl6](2), 1[SbCl6](2); is also obtained by SbCl5 oxidation of the 19-electron complex [(FeCp)-Cp-I*(C6Me6)], 1, at -80 degrees C. The 17-electron complexes are characterized by elemental analyses, ESR, Mossbauer, and UV/vis spectra, magnetic susceptibility, cyclic voltammetry, and quantitative single-electron reduction by ferrocene. The complex 1[SbCl6](2) is used as a very strong single-electron oxidant to also oxidize [Ru(bpy)(3)][PF6](2) to the 17-electron Rum species and the neutral cluster [FeCp(mu(3)-CO)](4) to its mono- and dications. The complex [(FeCp)-Cp-II(C6Me6)][PF6] is a redox catalyst for the anodic oxidation of furfural on Pt in SO2 via the Fe-II/Fe-III redox system. Density functional theory (DFT) calculations on various 17-electron compounds [Fe(C5R5)(C6R6)](+/2+) (R = H, Me) and [FeCp(C6H5NH2)](2+), as well as on the isoelectronic complexes ferrocenium and [Fe(C6H6)(2)](3+) and their 18-electron parents, allowed a detailed comparison of the electronic structure, bonding, UV-visible spectra, and ionization potentials of these species. Although the nature of the HOMO is not always the same within the series of their 18-electron parents, all the computed 17-electron complexes have the same E-2(2) ground state corresponding to the metallic (a(1))(2)(e(2))(3) electron configuration. Full geometry optimizations lead to the prediction of their molecular structures for the lowest E-2(2) and (1)A(1) states.