Thermal Activation of Methane by [HfO](.+) and [XHfO](+) (X=F, Cl, Br, I) and the Origin of a Remarkable Ligand Effect.

The thermal reactions of methane with [HfO](.+) and [XHfO](+) (X=F, Cl, Br, I) were investigated by using FT-ICR mass spectrometry complemented by high-level quantum chemical calculations. Surprisingly, in contrast to the inertness of [HfO](.+) towards methane, the closed-shell oxide ions [XHfO](+) (X=F, Cl, Br) activate the H3 C-H bond to form the insertion products [Hf(X)(OH)(CH3 )](+) . The possible origin of this remarkable ligand effect is discussed.