New features in the redox coordination chemistry of metal nitrosyls {M–NO+; M–NO; M–NO−(HNO)}

We describe new developments in the coordination chemistry of bound nitrosyl, considering its three formal redox states: NO+, NO and NO−/HNO. We emphasize on the correlation between well disclosed structural and spectroscopic aspects and different reactivity properties associated with the total electron content, according to the {MNO}n description (n = 6, 7, 8 for the above mentioned nitrosyls, respectively). The selected systems contain mainly six-coordinated nitrosyl-complexes with different MLx fragments (M = Fe, Ru, Os; L = cyanides, polypyridines, amines, EDTA, porphyrins, etc.). We focus heavily on the pentacyanonitrosylferrate systems, though with an eye toward a generalized description. For the NO+-complexes (n = 6), the electrophilic reactivity toward selected nucleophiles: OH−, N2H4, NO2− and cysteine is analyzed. We provide a mechanistic analysis, including DFT calculations for describing the reactants, transition states, intermediates and products. The crucial role of the redox potential associated to the NO+/NO couples, ENO+/NOENO+/NO, in determining the electrophilic addition reactivities is highlighted. We employ a similar approach for studying the nucleophilic reactivity of NO-complexes (n = 7) toward O2, a reaction that has great biological significance. Finally, some recent results covering structural, spectroscopic and reactivity aspects of NO− and HNO-complexes (n = 8) are reviewed.