Re-backbonding

Chart 11.5 Generic depiction (on the left-hand side) of metal-to-aromatic re-backbonding with benzene used as the aromatic substrate (arrows denote flow of electron density from the metal dre orbital to the aromatic C=C re molecular orbital). On the right-hand side, conversion of T)2-benzene complex to phenyl hydride complex, via C—H oxidative addition, restores aromaticity. 

Does the phosphine group function simply as a a donor, or is there a significant amount of Re 5d — P 3d 7r backbonding ... 

Re 5cf — P 3cf 7r backbonding is a minor effect compared with P 3p — Re 5d a donation. The net transfer of negative charge to PH3 largely results from Re-H — Pa donor interactions. 

This change of character of the MLCT transition has been observed before for the mononuclear d6-complexes MX(CO)3(a-diimine)(M=Mn,Re X=Cl,Br) and M(CO)i,(a-diimine) (M=Cr,Mo,W) and ascribed to a change of interaction (tr-backbonding) between the d M) and tr -orbitals involved in this transition (3). When the tr-backbonding increases, the transition loses its... 

The complexes can be both oxidized and reduced reduction potentials for many of the complexes are shown in Table 6. Cyclic voltammograms of Re(a-diimine)(CO)3X show that in most cases the first oxidation is chemically irreversible at scan rates of 0.1-0.2 V s however, at much faster sweep rates (>100 V s ) a reversible wave is observed at 1.32 V (vs. SCE) in MeCN for Re(bpy)(CO)3Cl [60]. The first oxidation is metal based and is followed by the rapid loss of carbon monoxide due to the weakening of the Re 7r-backbonding... 

IR spectral changes during the further reduction of Re(bpy)(CO)3Cl indicate rapid loss of a Cl ligand with formation of [Re(bpy)(CO)3] . The further reduction of either [Re(bpy)(CO)3(solvent)] or [Re(bpy)(CO)3]2 also yields [Re(bpy)(CO)3]. This illustrates the instability of non-Ti-backbonding ligands coordinated to the formally Re center. 

Reaction mechanism Based on the observation of reaction intermediates in the crystal structure and on quantum chemical calculations Einsle et al. [148] propose an outline of the first detailed reaction mechanism of the cytochrome c Nir from W. succinogenes. Nitrite reduction starts with a het-erolytic cleavage of the weak N-O bond, which is facilitated by a pronounced backbonding interaction between nitrite and the reduced active site iron. The protons come firom a highly conserved histidine and tyrosine. Elimination of one of both amino acids results in a significant reduced activity. Subsequently, two rapid one-electron reductions lead to a FeNO form and, by protonation, to a HNO adduct. A further two-electron two-proton step leads to hydroxylamine. The iron in the hydroxylamine complex is in the Fe(III) state [149], which is unusual compared to synthetic iron-hydroxylamine complexes where the iron is mainly in the Fe(II) state. Finally, it readily loses water to give the product, ammonia. This presumably dissociates firom the Fe(III) form of the active site, whose re-reduction closes the reaction cycle. 

However, while RE carbonyl complexes are not bottleable, they have been detected under matrix isolation conditions in frozen argon matrixes or in CO atmosphere high-pressure studies. Binary RE carbonyls, [M(CO) ] (n = 1-6), have been observed with some evidence for the formation of M(CO) (n = 7 or 8) species. These binary RE carbonyl adducts typically exhibit a single carbonyl stretch which is 145 195 cm lower than the stretching frequency of 2134 cm for free CO. Backbonding could be assumed to operate, which is reasonable for RE metals in a formal zero-oxidation state where valence orbitals are more chemically accessible. However, it has been noted that the infrared (IR) spectra of all binary RE carbonyls across the RE series... 

A well-developed series of complexes with rich MLCT excited-state behaviour are Re(I)-diimine complexes. [Re(bpy)(CO)3Cl] was the first transition metal complex used as a catalyst for CO2 reduction to CO, proposed by Lehn and Ziessel [41]. This series of complexes is particularly amenable to study of the excited state by time-resolved infrared spectroscopy. Formation of the MLCT Re bpy excited state leads to a reduction of the electron density on the metal centre. Consequently, d n backbonding from Re ti-orbitals to the antibonding n orbitals of CO ligands is reduced, resulting in an increase of the energy of the stretching vibrations, v(CO), by several tens of wavenumbers in the excited state if... 

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