Density functional theory (DFT) and multiconfiguration self-consistent field (MCSCF) calculations on the oxo FeO42- (Fe-VI) and the hypothetical oxo FeO4- (Fe-VII), and FeO4 (Fe-VIII) and peroxo FeO2(O-O)(z) [z = -2 (Fe-IV), z = -1 (Fe-V), z = 0 (Fe-VI)], Fe(O-O)(2)(z) [z = -2 (Fe-II), z = -1 (Fe-III), z = 0 (Fe-IV)], and FeO(O-O)(2)(z) [z = -2 (Fe-IV), z = -1 (Fe-V), z = 0 (Fe-VI)] clusters are presented and discussed. The results show the potential of stabilizing Fe-VII and Fe-VIII in tetrahedral oxo coordination. On the basis of absolute electronegativities calculated using DFT, it is predicted that FeO4 will be rather oxidizing, even stronger than Ch and O-2. On the basis of a comparison between total bonding energies of (M1M2FeO4)-O-VI (M-1, M-2 = Li, K), (MFeO4)-O-VII (M = Li, K), and (FeO4)-O-VI clusters, possible synthetic routes for electrochemical preparation of FeO4- and FeO4 species are discussed.