Properties of {FeNO}⁸ and {CoNO}⁹ metal nitrosyls in relation to nitroxyl coordination chemistry

While nitric oxide (NO) is an important signaling molecule and contributes to important physiological processes in humans, the one-electron reduced analogue of NO, namely nitroxyl (NO^- or HNO), is quickly establishing its own unique biological role. For example, HNO has been demonstrated to increase myocardial contractility by interacting with thiols in receptor molecules of heart muscle cells. These types of properties have made this particular nitrogen oxide molecule an attractive target for certain therapeutics. Since most of the underlying chemical biology of nitroxyl is mediated by heme proteins and Fe-nitroxyl intermediates have been proposed as key intermediates in denitrifying enzymes, several iron-porphyrin-NO coordination complexes of the Enemark-Feltham {FeNO}⁸ notation have been pursued. Herein we describe the collective efforts on the synthetic, structural, spectroscopic, electrochemical, and theoretical work that have been performed on biologically relevant iron- and cobalt-NO complexes that afford the rare {FeNO}⁸ and {CoNO}⁹ notations. This compilation has provided a somewhat unifying picture into the electronic structure of this important {MNO} unit as well as the benchmark properties that will enable the bioinorganic community to characterize and determine the fate of these species in biology, especially at the active sites of metalloenzymes involved in the global nitrogen cycle.