Iridium forms compound in +9 oxidation state

An international team of scientists has made a compound containing iridium in the +9 oxidation state – something that has been predicted by theoretical models but never formed experimentally before.

Until now, the highest oxidation state any element was shown to exist at was +8. Iridium, with nine valence electrons, can exist in a variety of oxidation states – the most common under normal conditions are +3 and +4. In 2009, researchers made molecules of IrO₄ – a form of iridium oxide where iridium was formally in the +8 state, with an outer shell electron configuration of 5d¹. Subsequent theoretical models suggested the last d-orbital electron could be ‘removed’ to create a stable iridium oxide cation where iridium could be counted as being in the +9 oxidation state.

Now, a team led by Mingfei Zhou at Fudan University in China have successfully formed the [IrO₄]⁺ cation in the gas phase using pulsed-laser vaporisation of an iridium metal target, and identified it using photodissociation spectroscopy.

The team also carried out calculations to predict some of the salts [IrO₄]⁺ might be able to form, but have so far not managed to prepare any of these.