The 18-electron complexes [MII(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 MIII 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 [FeII2(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 [FeIICp*(arene)][EX6] (EX6 = PF6 or SbCl6) is achieved by using SbCl5 in CH2Cl2 at 20 °C or SbF5 in SO2 at −10 °C or by Br2 + [Ag][SbF6] and gives the purple 17-electron complexes [FeIIICp*(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 [FeIIICp*(C6Me6)][SbCl6]2, 1[SbCl6]2, is also obtained by SbCl5 oxidation of the 19-electron complex [FeICp*(C6Me6)], 1, at −80 °C. The 17-electron complexes are characterized by elemental analyses, ESR, Mössbauer, 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 RuIII species and the neutral cluster [FeCp(μ3-CO)]4 to its mono- and dications. The complex [FeIICp(C6Me6)][PF6] is a redox catalyst for the anodic oxidation of furfural on Pt in SO2 via the FeII/FeIII 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 2E2 ground state corresponding to the metallic (a1)2(e2)3 electron configuration. Full geometry optimizations lead to the prediction of their molecular structures for the lowest 2E2 and 1A1 states.