The concept of formal oxidation states is fundamental to our understanding of general chemistry and is enshrined in the periodic table that groups elements principally by atomic number and electron configuration. The preparation and characterization of compounds containing elements with unusual oxidation states is of therefore of great interest to inorganic chemists. The highest experimentally known formal oxidation state of any chemical element is currently VIII. Here, the authors report the formation and infrared photodissociation spectroscopic identification of the iridium tetroxide cation, [IrO4]+, in which the iridium centre has a formal oxidation state of IX, the highest oxidation state known so far.
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