title: Identification of an iridium-containing compound with a formal oxidation state of IX.
abstract: One of the most important classifications in chemistry and within the periodic table is the concept of formal oxidation states. The preparation and characterization of compounds containing elements with unusual oxidation states is of great interest to chemists. The highest experimentally known formal oxidation state of any chemical element is at present VIII, although higher oxidation states have been postulated. Compounds with oxidation state VIII include several xenon compounds (for example XeO4 and XeO3F2) and the well-characterized species RuO4 and OsO4 (refs 2-4). Iridium, which has nine valence electrons, is predicted to have the greatest chance of being oxidized beyond the VIII oxidation state. In recent matrix-isolation experiments, the IrO4 molecule was characterized as an isolated molecule in rare-gas matrices. The valence electron configuration of iridium in IrO4 is 5d(1), with a formal oxidation state of VIII. Removal of the remaining d electron from IrO4 would lead to the iridium tetroxide cation ([IrO4](+)), which was recently predicted to be stable and in which iridium is in a formal oxidation state of IX. There has been some speculation about the formation of [IrO4](+) species, but these experimental observations have not been structurally confirmed. Here we report the formation of [IrO4](+) and its identification by infrared photodissociation spectroscopy. Quantum-chemical calculations were carried out at the highest level of theory that is available today, and predict that the iridium tetroxide cation, with a Td-symmetrical structure and a d(0) electron configuration, is the most stable of all possible [IrO4](+) isomers.
authors.affiliation: Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials, Fudan University, Shanghai 200433, China.
authors.forename: Guanjun
authors.initials: G
authors.lastname: Wang
authors.affiliation: Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials, Fudan University, Shanghai 200433, China.
authors.forename: Mingfei
authors.initials: M
authors.lastname: Zhou
authors.affiliation: Department of Chemistry, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4M1, Canada.
authors.forename: James T
authors.initials: JT
authors.lastname: Goettel
authors.affiliation: Department of Chemistry, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4M1, Canada.
authors.forename: Gary J
authors.initials: GJ
authors.lastname: Schrobilgen
authors.affiliation: Department of Chemistry and Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Tsinghua University, Beijing 100084, China.
authors.forename: Jing
authors.initials: J
authors.lastname: Su
authors.affiliation: Department of Chemistry and Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Tsinghua University, Beijing 100084, China.
authors.forename: Jun
authors.initials: J
authors.lastname: Li
authors.affiliation: Institut für Anorganische und Analytische Chemie, Albert-Ludwigs Universität Freiburg, Albertstrasse 21, D-79104 Freiburg im Breisgau, Germany.
authors.forename: Tobias
authors.initials: T
authors.lastname: Schlöder
authors.affiliation: 1] Institut für Anorganische und Analytische Chemie, Albert-Ludwigs Universität Freiburg, Albertstrasse 21, D-79104 Freiburg im Breisgau, Germany [2] Institut für Chemie und Biochemie - Anorganische Chemie, Freie Universität Berlin, Fabeckstrasse 34-36, D-14195 Berlin, Germany.
authors.forename: Sebastian
authors.initials: S
authors.lastname: Riedel