X-ray absorption spectroscopy of a coordinatively unsaturated 3d transition metal complex

Coordinatively unsaturated transition-metal complexes are employed as active catalysts in a wide range of homogeneous chemical reactions including C–H bond activation. Because of their high reactivity and resulting affinity to rapidly bind substrate molecules, they are generally challenging to detect and characterize when prepared photochemically. Here, we generate the 16-electron cationic complex [CpFe(CO)2]+ (Cp = cyclopentadienyl) in the gas phase using electrospray ionization and probe its electronic structure with X-ray absorption spectroscopy at the Fe L-edge. Supported by multiconfigurational spectrum calculations, the distinct L-edge absorption profile of [CpFe(CO)2]+ reveals direct access to a low-lying unoccupied Fe 3d-derived orbital, which is characteristic of the unsaturated coordination of the complex and of its pronounced affinity to bind even notoriously unreactive moieties such as alkanes.