Metal–dioxygen and metal–dinitrogen complexes: where are the electrons?

Abstract

Transition-metal complexes of O2 and N2 play an important role in the environment, chemical industry, and metalloenzymes. This Perspective compares and contrasts the binding modes, reduction levels, and electronic influences on the nature of the bound O2 or N2 group in these complexes. The charge distribution between the metal and the diatomic ligand is variable, and different models for describing the adducts have evolved. In some cases, single resonance structures (e.g. M-superoxide = M–O2−) are accurate descriptions of the adducts. Recent studies have shown that the magnetic coupling in certain N22− complexes differs between resonance forms, and can be used to distinguish experimentally between resonance structures. On the other hand, many O2 and N2 complexes cannot be described well with a simple valence-bond model. Defining the situations where ambiguities occur is a fertile area for continued study.

The Mechanism of N–N Double Bond Cleavage by an Iron(II) Hydride Complex

“…23 The magnetism [L Me,Et V] 2 similarly indicated weak coupling ( J = −0.8 cm −1 ) of two S = 1 V(I) ions. 27 …”

Structural, spectroscopic and theoretical studies of a diruthenium(II,II) tetraformamidinate that reversibly binds dioxygen

“…Binding of dioxygen by metal complexes has received significant attention from synthetic chemists because of its relevance to the transport and activation of molecular oxygen in biological systems [20], and the insight it provides into possible mechanisms for oxidation catalysts [21]. As observed previously for diruthenium(II,II) tetraformamidinates [9], solutions of purple solutions decompose within an hour at room temperature, and within 8 hours at -18°C, to…”

Coordination chemistry insights into the role of alkali metal promoters in dinitrogen reduction

“…Specifically, the stretching frequency of the N-N bond (ν NN , 2359 cm −1 for unbound N 2 ) allows for the direct assessment of the strength of the N-N bond (Figure 6), with lower frequencies indicating a higher degree of reduction (due to more electron density in the π-antibonding orbitals of N 2 ) [82]. Similarly, in a solid-state crystal structure the distance between N atoms can be measured to give another estimation of N 2 reduction (1.098 Å for atmospheric N 2 ), as longer distances imply lower bond order.…”