Re-examination of the formation of dinitrosyl–iron complexes during reaction of S-nitrosothiols with Fe(II)

The reaction of S-nitrosothiol compounds with ferrous ions in solution has been investigated and the generated dinitrosyl–iron complexes have been characterized. During the reaction of S-nitrosocysteamine with Fe(II) in water solution in the presence of a twofold excess (with respect to iron) of cysteamine hydrochloride (CSH), an EPR-silent dinuclear iron complex (complex A of formula [Fe2(RS)2(NO)4]) was formed as the major species and was characterized by FAB MS+, UV–Vis, NMR and IR spectroscopies. In the presence of a large excess of CSH (CSH/Fe(II)=20:1), a green paramagnetic mononuclear complex (complex B of formula [Fe(RS)2(NO)2]−) was formed. From EPR and UV–Vis data, and also on the basis of the few crystallographic structures known for similar complexes, complex B is proposed to display a distorted tetrahedral geometry (C2v), approaching a trigonal bipyramid with a missing ligand, with the unpaired electron mainly localized on the dz2 orbital of the iron characterized by a d9 electronic configuration.