Identification of a Two‐Coordinate Iron(I)–Oxalate Complex

Exotic oxidation states of the first‐row transition metals have recently attracted much interest. In order to investigate the oxidation states of a series of iron–oxalate complexes, an aqueous solution of iron(III) nitrate and oxalic acid was studied by infrared free liquid matrix‐assisted laser desorption/ionization as well as ionspray mass spectrometry. Here, we show that iron is not only detected in its common oxidation states +II and +III, but also in its unusual oxidation state +I, detectable in both positive‐ion and in negative‐ion modes, respectively. Vibrational spectra of the gas phase anionic iron oxalate complexes [Fe(III)(C(2)O(4))(2)](−), [Fe(II)(C(2)O(4))CO(2)](−), and [Fe(I)(C(2)O(4))](−) were measured by means of infrared photodissociation spectroscopy and their structures were assigned by comparison to anharmonic vibrational spectra based on second‐order perturbation theory.