Structure and UV-vis spectroscopy of roussin black salt $[Fe_4S_3(NO)_7]^{-}$

The calculations of the electronic structure, geometry and electronic spectra of the Roussin's black salt anion (RBS) were carried out with the RB3LYP and UB3LYP methods. The geometry was determined by the RB3LYP method. The electronic structure of RBS may be described as composed of the one ${Fe(NO)}^{7}$ unit, in which ferric ion (S=5/2) is antiferromagnetically coupled to a $NO^{-}$ ligand (S=1) giving S=3/2; such a unit is antiferromagnetically coupled to three ${Fe(NO)_{2}}^{9}$ units, each of them is formed by a ferric ion (S=5/2) antiferromagnetically coupled to two $NO^{-}$ ligands with S=1. The resulting total spin of RBS is S=0. The $S^{2-}$ bridges mediate in the antiferromagnetic coupling.The calculated spectrum of RBS shows agreement with the experimental spectra. The calculated transitions are mainly of charge transfer character. The photochemical behaviour is interpreted.