Credit: ©2008 NPG

Oxygen is crucial to many biological and chemical processes, some of which involve transition metals. Terminal oxo complexes, with a single oxygen atom directly bonded to the metal, are believed to be key intermediates in reactions as important as the photocatalytic oxidation of water to evolve O2. Although oxo complexes of early transition metal complexes are known, the extra d-electrons in later metals — such as the important catalysts platinum and palladium — have a destablizing effect. This can be countered by attaching electron-accepting ligands, but this is not always feasible or desirable.

David Milstein and colleagues at the Weizmann Institute in Israel have now prepared1 a d6 platinum terminal oxo complex that does not need to be stabilized by electron-accepting groups. The existence of the elusive Pt=O bond was confirmed by a combination of structural and computational studies, and X-ray spectroscopy data showed that the platinum was in the 4+ oxidation state. The geometry of the complex is distorted out of regular square-planar by around 35°, and this non-planarity is believed to reduce the overlap between strongly anti-bonding orbitals and thus stabilize the system.

The complex can act as an oxygen transfer agent, reacting with carbon monoxide to give carbon dioxide, for example. Addition of water created a new complex with platinum bonded to two hydroxyl groups and one water molecule, supporting those models that suggest that terminal oxo complexes are intermediates in catalytic water oxidation. The existence of these elusive compounds goes some way to justifying their implication as intermediates in many catalytic reactions.