Mono- and dinuclear non-heme iron–nitrosyl complexes: Models for key intermediates in bacterial nitric oxide reductases

High-spin non-heme iron–nitrosyls are of direct interest to both the chemical and biological communities as these species exhibit interesting chemical properties and act as direct models for enzymatic intermediates. The electronic ground state of the ferrous NO complexes, {Fe–NO}7, is best described as high-spin FeIII antiferromagnetically coupled to NO−, generating the spectroscopically observed S = 3/2 ground state. These species have been identified as catalytically relevant to a variety of NO-reducing enzymes such as bacterial nitric oxide reductase (NorBC) and flavo(rubredoxin) nitric oxide reductase (FNOR). Recently, the corresponding one-electron reduced {Fe–NO}8 (nitroxyl) complexes have also been implicated as biologically significant species. In this review the available spectroscopic data for {Fe–NO}7 and {Fe–NO}8 mono- and dinuclear non-heme iron–nitrosyls are summarized, and the implications of these results with respect to the electronic structures and reactivities of these species, in particular towards NO reduction, are discussed.

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► Non-heme high-spin {Fe–NO}7 complexes show S = 3/2 ground states where a high-spin Fe(III) is antiferromagnetically coupled to the bound NO− ligand.
► In these complexes, NO− acts mostly as a π donor ligand. The strength of this π bond is modulated by the effective nuclear charge of iron, as evident from a direct correlation of the Fe–NO and N–O stretching frequencies in a series of these compounds.
► The FeNO bond is very covalent in these complexes, such that the bound NO− ligand is not basic, and does not show radical-type reactivity.
► One-electron reduced non-heme high-spin {Fe–N(H)O}8 complexes are postulated as intermediates in bacterial respiratory NO reductases, but the properties of these complexes are not well defined.
► Dinuclear non-heme high-spin ({Fe–NO}7)2 complexes are key intermediates in bacterial scavenging NO reductases.