The ubiquitous function of nitric
oxide (NO) guided the biological
discovery of the natural dinitrosyliron unit (DNIU) [Fe(NO)2] as an intermediate/end product after Fe nitrosylation of nonheme
cofactors. Because of the natural utilization of this cofactor for
the biological storage and delivery of NO, a bioinorganic study of
synthetic dinitrosyliron complexes (DNICs) has been extensively explored
in the last 2 decades. The bioinorganic study of DNICs involved the
development of synthetic methodology, spectroscopic discrimination,
biological application of NO-delivery reactivity, and translational
application to the (catalytic) transformation of small molecules.
In this Forum Article, we aim to provide a systematic review of spectroscopic
and computational insights into the bonding nature within the DNIU
[Fe(NO)2] and the electronic structure of different types
of DNICs, which highlights the synchronized advance in synthetic methodology
and spectroscopic tools. With regard to the noninnocent nature of
a NO ligand, spectroscopic and computational tools were utilized to
provide qualitative/quantitative assignment of oxidation states of
Fe and NO in DNICs with different redox levels and ligation modes
as well as to probe the Fe–NO bonding interaction modulated
by supporting ligands. Besides the strong antiferromagnetic coupling
between high-spin Fe and paramagnetic NO ligands within the covalent
DNIU [Fe(NO)2], in polynuclear DNICs, the effects of the
Fe···Fe distance, nature of the bridging ligands, and
type of bridging modes on the regulation of the magnetic coupling
among paramagnetic DNIU [Fe(NO)2] are further reviewed.
In the last part of this Forum Article, the sequential reaction of
{Fe(NO)2}10 DNIC [(NO)2Fe(AMP)] (1-red) with NO(g), HBF4, and KC8 establishes a synthetic cycle, {Fe(NO)2}9-{Fe(NO)2}9 DNIC [(NO)2Fe(μ-dAMP)2Fe(NO)2] (1) → {Fe(NO)2}9 DNIC [(NO2)Fe(AMP)][BF4] (1-H) → {Fe(NO)2}10 DNIC 1-red → DNIC 1, for the transformation
of NO into HNO/N2O. Of importance, the NO-induced transformation
of {Fe(NO)2}10 DNIC 1-red and [(NO)2Fe(DTA)] (2-red; DTA = diethylenetriamine) unravels
a synthetic strategy for preparation of the {Fe(NO)2}9-{Fe(NO)2}9 DNICs [(NO)2Fe(μ-NHR)2Fe(NO)2] containing amido-bridging ligands, which
hold the potential to feature distinctive physical properties, chemical
reactivities, and biological applications.