Transition-metal oxo complexes with multiply bonded ligands have low stability if their M=O antibonding orbitals are populated. Electron-rich compounds are considered key transient species in catalytic oxo transfer reactions. However, beyond group 8 metals, few well-characterized examples are known.

Sven Schneider, Universität Göttingen, Germany, Max C. Holthausen, Goethe-Universität Frankfurt, Germany, and colleagues have discovered an iridium complex (group 9) with a terminal oxo ligand stabilized by a bulky pincer framework (pictured above). The IrO{N(CHCHPtBu₂} complex is synthesized by either deprotonation with potassium tert-butoxide or hydrogen atom transfer with 2,4,6-tris-tert-butylphenoxyl (pictured below). The complex possesses an unprecedented triplet ground state with two unpaired electrons delocalized equally within the Ir=O core.

This unusual bonding situation allows ambiphilic oxo transfer to nucleophiles, such as PMe₃, and electrophiles, such as CO₂. Calorimetric and computational examination of the bond dissociation energy of the IrO–H bond suggests that the oxo complex could activate strong C–H bonds, such as those found in hydrocarbons.

• A Terminal Iridium Oxo Complex with a Triplet Ground State,
  Daniel Delony, Markus Kinauer, Martin Diefenbach, Serhiy Demeshko, Christian Würtele, Max C. Holthausen, Sven Schneider,
  Angew. Chem. Int. Ed. 2019, 58, 10971–10974.
  https://doi.org/10.1002/anie.201905325