Dinitrosyl Iron Complexes with Thiol-Containing Ligands in Aqueous Solution are Presented in their Cationic form with the d7 Electronic Configuration of Iron (Review)

Abstract

It is assumed that the existence of dinitrosyl iron complexes (DNIC) with thiol-containing ligands in paramagnetic cationic or anionic forms (with d⁷ or d⁹ electron configurations of iron, respectively, described in the Enemark-Feltman representation by the formulas [Fe(NO)₂]⁷ and [Fe(NO)₂]⁹) is determined by the mechanism of formation of these complexes. The cationic form arises as a result of the disproportionation reaction of two NO molecules bound to the Fe²⁺ ion, whereas the anionic form of DNIC is formed as a result of one-electron reduction of the mononitrosyl Fe²⁺ complex by one of the two thiol-containing ligands, followed by the addition of a second NO molecule to the complex. In the absence of solvent, both forms of DNIC are characterized by a tetrahedral spatial structure. When dissolved in water, the cationic form, incorporating two water molecules, acquires a four-coordinated square-planar structure, the upper molecular orbital of which is characterized by the inclusion of the d_z2 orbital of iron. The mononuclear cationic form of DNIC with thiol-containing ligands and a predominant resonance structure [(RS⁻)₂Fe²⁺(NO)(NO⁺)]⁺ is capable of acting in living organisms as donors of both NO molecules and nitrosonium cations.