The History and the Current Revival of the Oxo Chemistry of Iron in its Highest Oxidation States: Fe^VI – Fe^VIII

The history of the oxo compounds of iron in its highest oxidation states is reviewed and modern activities in this long neglected area of inorganic chemistry are highlighted. The chemistry of ferrates(VI) is the most rapidly advancing branch owing to several potential applications in diverse fields such as environmental chemistry and energy storage. Convenient and high-yield preparations of ferrates(VI) in high purity are presented, followed by a coverage of the analytical, spectroscopic, and structural characterization in the solid and in solution, with a focus on the stability of these compounds, which had long been under-estimated. Particular attention has been paid to the fascinating mechanisms that have been proposed for the intriguing “self-decay” of the [FeO₄]^2– dianion. Redox processes with inorganic and organic substrates are summarized including fresh and waste water treatment on the one hand and “super-iron batteries” on the other. Recent advances in the experimental and computational approach to ferrates(VII) [FeO₄]^– and the elusive “iron tetroxide” [FeO₄] are described.