Non-innocent ligands in bioinorganic chemistry—An overview

The potential of redox-active ligands to behave “noninnocently” in transition-metal coordination compounds is reflected with respect to various aspects and situations. These include the question of establishing “correct” oxidation states, the identification and characterization of differently charged radical ligands, the listing of structural and other consequences of ligand redox reactions, and the distinction between barrierless delocalized “resonance” cases Mn/Ln ↔ Mn+1Ln–1 versus separated valence tautomer equilibrium situations Mn/Ln ⇌ Mn+1Ln–1. Further ambivalence arises for dinuclear systems with radical bridge Mn(μ-L•)Mn versus mixed-valent alternatives Mn+1(μ-L–)Mn, for noninnocent ligand-bridged coordination compounds of higher nuclearity such as (μ3-L)M3, (μ4-L)M4, (μ-L)4M4, or coordination polymers. Conversely, the presence of more than one noninnocently behaving ligand at a single transition-metal site in situations such as Ln–M–Ln–1 or L•–M–L• may give rise to corresponding ligand-to-ligand ...

Manifestations of noninnocent ligand behavior.

The nitrate anion is a simple, abundant and relatively stable species, yet plays a significant role in global cycling of nitrogen, global climate change, and human health. Although it has been known for quite some time that nitrate is an important species environmentally, recent studies have identified potential medical applications. In this respect the nitrate anion remains an enigmatic species that promises to offer exciting science in years to come. Many bacteria readily reduce nitrate to nitrite via nitrate reductases. Classified into three distinct types – periplasmic nitrate reductase (Nap), respiratory nitrate reductase (Nar) and assimilatory nitrate reductase (Nas), they are defined by their cellular location, operon organization and active site structure. Of these, Nap proteins are the focus of this review. Despite similarities in the catalytic and spectroscopic properties Nap from different Proteobacteria are phylogenetically distinct. This review has two major sections: in the first section, nitrate in the nitrogen cycle and human health, taxonomy of nitrate reductases, assimilatory and dissimilatory nitrate reduction, cellular locations of nitrate reductases, structural and redox chemistry are discussed. The second section focuses on the features of periplasmic nitrate reductase where the catalytic subunit of the Nap and its kinetic properties, auxiliary Nap proteins, operon structure and phylogenetic relationships are discussed.

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Freya J. Hine

Papers

Dithiolene complexes and the nature of molybdopterin