Muonium Chemistry at Diiron Subsite Analogues of [FeFe]-Hydrogenase

The chemistry of metal hydrides is implicated in a range of catalytic processes at metal centers. Gaining insight into the formation of such sites by protonation and/or electronation is therefore of significant value in fully exploiting the potential of such systems. Here, we show that the muonium radical (Mu.), used as a low isotopic mass analogue of hydrogen, can be exploited to probe the early stages of hydride formation at metal centers. Mu. undergoes the same chemical reactions as H. and can be directly observed due to its short lifetime (in the microseconds) and unique breakdown signature. By implanting Mu. into three models of the [FeFe]‐hydrogenase active site we have been able to detect key muoniated intermediates of direct relevance to the hydride chemistry of these systems. New radicals: Interaction of muonium with {2Fe2S} cores leads to the formation of mixed‐valence adducts. Detection of these species opens a new window on the reactivity of hydride at redox‐active metal centers. The unique selectivity and timescale of muon spin spectroscopy has the potential to reveal reactivity in a wide range of catalytic intermediates.