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

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(H₂ 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 [FeCl₃ (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 [FeCl₃ (NO)]^- ion and related species such as the [FeCl₂ (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.