{FeNO}⁷‐Type Halogenido Nitrosyl Ferrates: Syntheses, Bonding, and Photoinduced Linkage Isomerism

Abstract

Abstract Mononitrosyl–iron compounds (MNICs) of the Enemark–Feltham {FeNO} 7 type can be divided into a doublet ( S =1/2) and a quartet ( S =3/2) spin variant. The latter relies on weak‐field co‐ligands such as amine carboxylates. Aqua‐only co‐ligation appears to exist in the long‐known “brown‐ring” [Fe(H 2 O) 5 (NO)] 2+ cation, which was prepared originally from ferrous salts and NO in sulfuric acid. A chloride variant of this species, the green [FeCl 3 (NO)] − ion, was first prepared analoguosly by using hydrochloric instead of sulfuric acid. As a tetrahedral species, it is the simple prototype of sulfur‐bonded {FeNO} 7 ( S =3/2) MNICs of biological significance. Although it has been investigated for more than a century, neither clean preparative routes nor reliable structural parameters were available for the [FeCl 3 (NO)] − ion and related species such as the [FeCl 2 (NO) 2 ] − ion, a prototypical dinitrosyliron species (a “DNIC”). In this work, both issues have been resolved. In addition, we report on a computational study on the ground‐ and excited‐state properties including an assignment of the chromophoric transitions. Photoinduced metastable isomers were characterised in a combined experimental and computational approach that resulted in the confirmation of a single photoinduced linkage isomer of the paramagnetic nitrosyl–metal coordination entity.