Three redox states of metallonitrosyls in aqueous solution

Abstract

This contribution deals with the structure and reactivity of bound nitrosyl in transition-metal centers (group 8: Fe, Ru, Os). The focus is set on pseudooctahedral nitrosyl-complexes with coordination number 5 and 6, containing ancillary coligands of both heme- and nonheme type. The discussion is organized in terms of Enemark and Feltham's classification, selecting complexes within the {MNO}n framework (n=6, 7, and 8). The examples have been chosen for a best description of the electronic structures in terms of modern structural, spectroscopical, and computational methodologies. The selected {MNO}6,7,8 species reflect the occurrence of three redox states of bound nitrosyl, frequently (though not always) described as NO+, NO, and NO- for n=6, 7, and 8, respectively. The analysis is centered on the members of a series of complexes for which the three redox states have been observed on the same platform, viz., [Fe(CN)5(NO)]2,3,4- and [Ru(Me3[9]aneN3)(bpy)(NO)]3,2,1+, in aqueous solutions. The influence of the donor-acceptor character of the coligands is specifically addressed with emphasis on the ligand trans- to nitrosyl, showing that the latter group may exert a delabilizing influence (as NO+), as well as a labilizing one (NO-≫NO) on the trans-ligand. On the other hand, typical electrophilic reactivity patterns (toward different nucleophiles) are analyzed for M-NO+, and nucleophilic reactivity (with O2) is described for the reduced species, M-NO and M-(NO-). In the latter case, protonation is described by characterizing the bound HNO species. Important differences are highlighted in the chemistry of bound NO- and HNO, revealing the strong and mild reductant abilities of these species, respectively. The chemistry is analyzed in terms of the biological relevance to the behavior of nitrite- and NO-reductases and other NO-related enzymes.