Mononitrosyl-iron compounds (MNICs) of the Enemark-Feltham {FeNO} 7 type can be divided into ad oublet (S = 1/2) and aq uartet (S = 3/2) spin variant. The latter relies on weak-field co-ligands such as amine carboxylates. Aquaonly co-ligation appears to exist in the long-known "brownring" [Fe(H 2 O) 5 (NO)] 2 + cation, whichw as preparedo riginally from ferrous salts and NO in sulfuric acid. Ac hloride variant of this species, the green [FeCl 3 (NO)] À ion, was first prepared analoguosly by using hydrochloric instead of sulfuric acid. As at etrahedral species, it is the simple prototypeo fs ulfurbonded {FeNO} 7 (S = 3/2) MNICs of biological significance. Although it has been investigated form ore than ac entury, neitherc lean preparative routes nor reliable structuralp arameters were available for the [FeCl 3 (NO)] À ion and related speciess uch as the [FeCl 2 (NO) 2 ] À ion, ap rototypical dinitrosylirons pecies( a" DNIC"). In this work, both issues have been resolved. In addition, we report on ac omputational study on the ground-and excited-state properties including an assignment of the chromophoric transitions. Photoinduced metastable isomersw ere characterisedi nacombined experimental and computational approacht hat resulted in the confirmation of as ingle photoinduced linkage isomer of the paramagnetic nitrosyl-metal coordination entity.

The colourless solid NO(HSO4), known as “lead‐chamber crystals”, was investigated ever since its first preparation more than two centuries ago. Its overall ionic nature now is confirmed by X‐ray crystallography [Pna21, a = 7.3558(4), b = 6.8924(3), c = 7.7017(3) Å, Z = 4]. The next neighbours of the NO+ cations are four hydrogensulfate oxygen atoms, forming a distorted square at a distance of about 2.5 Å from the nitrogen atom. The square pattern next to the nitrogen atom is the most widespread coordination figure about an NO+ ion in a nitrosyl salt. Depending on the anion, the interaction goes along with a decrease of the N–O stretch's excitation energy.

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A conspicuous detail of the so‐called brown‐ring test (the analytical test on nitrate) is the reddish color of the bottom layer of concentrated sulfuric acid, which develops upon the bleeding of the brown layer into the acid. Crystals of the same color form from a solution of ferrous sulfate in concentrated sulfuric acid on saturation with gaseous nitric oxide. The structure of this H3O[{Fe(NO)(μ4‐SO4)(μ2‐SO4)0.5}n/n] (1a) is made up from infinite chessboard‐type layers with sulfur on the field junctions and Fe(NO) moieties below the black and above the white fields. An Fe–N–O angle of about 160° causes disorder in the tetragonal space group I4/mmm. A similar crystal pathology was found in the related [{Fe(MeOH)(NO)(μ4‐SO4)}n/n] (1b) in the same crystal class. A one‐dimensional coordination polymer is formed in crystals of a third compound that comprises the Fe(NO)O5 coordination pattern, namely the brown oxalato species [{Fe(H2O)(NO)(μ2‐ox)}n/n·H2O] (2). A still larger NO tilt of about 156° is not obscured by disorder in the triclinic crystals of 2.