Iridium Dressed To The Nines

Periodic Table: IrO4+ is the first molecule with an element in the +9 oxidation state

Steve Ritter

TAKE A NUMBER Iridium has a propensity to take on an extensive range of oxidation states, expanded to +9 with IrO4+.

TAKE A NUMBER
Iridium has a propensity to take on an extensive range of oxidation states, expanded to +9 with IrO4+.

Before this year, the highest observed oxidation state of an atom in a molecule had been +8, which occurs in only a few tetroxide compounds—RuO4, OsO4, IrO4, and XeO4. These are molecules in which the central metal atom has plenty of valence electrons to give away and the high charge can be stabilized by small, highly electronegative ligands such as fluorine or oxygen. Among the +8 molecules, IrO4 stands out because iridium still has one more 5d valence electron to give. An international team set out on a quest to develop a method to coax iridium to go all the way—and they met with success, preparing, preparing IrO4+, the first example of a molecule with an element in an oxidation state of +9 (Nature 2014, DOI: 10.1038/nature13795). “New, well-documented oxidation states are rarer than new elements,” said theoretical chemist Pekka Pyykkö of the University of Helsinki, in ¥, when commenting on the achievement. The discovery team was led by Sebastian Riedel of Albert Ludwigs University, in Freiburg, Germany; Mingfei Zhou of Fudan University, in Shanghai; Jun Li of Tsinghua University, in Beijing; and Gary J. Schrobilgen of McMaster University, in Hamilton, Ontario. To make IrO4+, Zhou’s group blasted an iridium metal target with a pulsed laser in an argon atmosphere spiked with O2. The team identified IrO4+ among the reaction products and studied it by mass spectrometry, infrared spectroscopy, and computational techniques. Schrobilgen’s group tried to synthesize an isolable IrO4+ salt by treating iridium oxides with strong oxidizing reagents, but the chemists haven’t found the right combination just yet.

 Calculated optimized structures and energetic ordering of three detected IrO4 isomers; IrO4+ with iridium in the +9 oxidation state (left) is the most energetically favored isomer. Credit: Adapted from Nature

Calculated optimized structures and energetic ordering of three detected IrO4 isomers; IrO4+. with iridium in the +9 oxidation state (left) is the most energetically favored isomer.
Credit: Adapted from Nature

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