Electron transfer metal-ligand reactions

Reduction of bismuth compounds could take place by reaction with polymer radicals propagating the depolymerization of polypropylene, either by electron transfer or ligand transfer which are typical redox reactions between alkyl radicals and metal compounds 59... 

But in metal complexes it appears that other processes such as solvent attack on the excited state, electron transfer, and ligand dissociation can lead to excited state deactivation before bond rupture can occur. As seen in the summary in Table III, dissociative cleavage of a multiple metal-metal bond remains to be accomplished. Also, upper excited state reaction is the rule in the multiple bonded systems. 

The tris-carbene ligand family with fac geometry points its three wingtip groups downwards around the metal shielding it effectively from the approach of any but small substrates. Its main application is therefore the activation of small molecules, including the activation of dioxygen and proton coupled electron transfer (PCET), a reaction normally performed by certain enzymes [70,71],... 

The products of these reactions may be short lived, but they often have characteristic absorption spectra that can be detected by pulse radiolysis. Subsequent reactions, such as electron transfer and ligand labilization, can be followed kinetically with the appropriate detection technique. Reviews of the spectra, kinetics and mechanisms of complexes in unusual and unstable oxidation states are available A compilation of rate constants for the reactions of metal ions in unusual valency states is available ... 

Metal-oxygen intermediates react with inorganic or organic substrates via various reaction pathways, such as oxygen atom transfer, hydrogen atom transfer, hydride transfer, electron transfer, proton-coupled electron transfer, free radical reactions, and others.14-16 The preferential reactivity pathways depend on the nature and oxidation state of the metal, the nuclearity of the complex, and the coordination mode and protonation state of coordinated oxygen-derived ligand(s). 

Complexes such as CpFefCO) " or MoL react by this mechanism - because they are 18-electron metal centers containing ligands that do not readily dissociate. As noted in Scheme 7.1, the reaction of alkyl iodides with CpFefCO) occurs through radical intermediates, most likely formed by initial outer-sphere electron transfer, but the reactions of alkyl bronudes and sulfonates occur by S 2 pathways. Reactions of neutral, coordinatively saturated complexes containing tightly boimd ligands, such as ds-Mo(CO)2(dmpe). also occur by an outer-sphere electron transfer mechanism (Equation 7.7). 

Another difficulty which is related to the potentially catalytic use of organometallics concerns the often enormous substitutional labilization of such systems after heterogeneous or homogeneous electron transfer. Typical textbook cases are the ligand exchange reactions of 18 valence electron (VE) complexes which can be accelerated by many orders of magnitude via one-electron oxidation (17 VE intermediates) or reduction (19 VE intermediates) [10,11]. It is shown below (Section 6) how even partial intramolecular electron transfer from ligands to metals can activate these for CO substitution. 

The widespread use of fast-reaction methods has opened up the study of new types of reaction and new chemical species, and has raised new questions about physical aspects of mechanisms. Examples of types of reaction which could not be studied without fast-reaction techniques include the following among reactions of labile metal ions, ligand substitution, solvent exchange, and electron-transfer among organic reactions, many proton-transfer... 

So far, research in this area has emphasized metal clusters, organometallics, and quantum size semiconductor clusters and superlattices. As experimental techniques become available, the dynamics of intrazeolite reactions, such as catalysis, ligand exchange, electron transfer and radical reactions, and polymerizations will be explored in more detail. As molecular sieves with ever larger pores are being discovered, the future potential to assemble and understand supramolecular structures is enormous. 

Metals of changing valency influence oxidation rates by complex formation. The ligands are either substrate or enzyme protein molecules or both. Electron distribution is altered as well in the center as in the ligands, providing us with a number of catalysts of graded reactivity. As a consequence of one electron transfer, metal ions of changing valency may initiate chain reactions, whereby the rate of the oxidative process is greatly increased. 

Oxidation—Reduction. Redox or oxidation—reduction reactions are often governed by the hard—soft base rule. For example, a metal in a low oxidation state (relatively soft) can be oxidized more easily if surrounded by hard ligands or a hard solvent. Metals tend toward hard-acid behavior on oxidation. Redox rates are often limited by substitution rates of the reactant so that direct electron transfer can occur (16). If substitution is very slow, an outer sphere or tunneling reaction may occur. One-electron transfers are normally favored over multielectron processes, especially when three or more species must aggregate prior to reaction. However, oxidative addition... 

The side chains of the 20 different amino acids listed in Panel 1.1 (pp. 6-7) have very different chemical properties and are utilized for a wide variety of biological functions. However, their chemical versatility is not unlimited, and for some functions metal atoms are more suitable and more efficient. Electron-transfer reactions are an important example. Fortunately the side chains of histidine, cysteine, aspartic acid, and glutamic acid are excellent metal ligands, and a fairly large number of proteins have recruited metal atoms as intrinsic parts of their structures among the frequently used metals are iron, zinc, magnesium, and calcium. Several metallo proteins are discussed in detail in later chapters and it suffices here to mention briefly a few examples of iron and zinc proteins. 

N2 recognized as a bridging ligand in ((NH3)5RuN2Ru(NH3)5] by D. F. Harrison, E. Weissterger, and H. Taute. (H. Taute, 1983 Nobel Prize for chemistry for his work on the mechanisms of electron transfer reactions especially in metal complexes ). 

Novel electron-transfer reactions mediated by alkali metals complexed with crown ethers as macrocyclic ligands 98ACR55. 

Transition metal salts trap carbon-centered radicals by electron transfer or by ligand transfer. These reagents often show high specificity for reaction with specific radicals and the rates of trapping may be correlated with the nucleophilicity of the radical (Table 5.6). For example, PS radicals are much more reactive towards ferric chloride than acrylic propagating species."07... 

Disiloxane, tetramesityl-, 3,206 Disproportionation iridium catalysts, 4,1159 Dissolution nuclear fuels, 6, 927 Distannene, 3,217 Distannoxane, 1,3-dichloro-, 3,207 Distibine, tetraphenyl-, 2,1008 Distibines, 2,1008 Disulfido ligands metal complexes, 2,531-540, 553 bonding, 2, 539 electron transfer, 2, 537 intramolecular redox reactions, 2,537 reactions, 2, 537... 

Finally, we consider the alternative mechanism for electron transfer reactions -the inner-sphere process in which a bridge is formed between the two metal centers. The J-electron configurations of the metal ions involved have a number of profound consequences for this reaction, both for the mechanism itself and for our investigation of the reaction. The key step involves the formation of a complex in which a ligand bridges the two metal centers involved in the redox process. For this to be a low energy process, at least one of the metal centers must be labile. 

Iron-sulfur proteins. In an iroinsulfiir protein, the metal center is surrounded by a group of sulfur donor atoms in a tetrahedral environment. Box 14-2 describes the roles that iron-sulfur proteins play in nitrogenase, and Figure 20-30 shows the structures about the metal in three different types of iron-sulfur redox centers. One type (Figure 20-30a l contains a single iron atom bound to four cysteine ligands. The electron transfer reactions at these centers... 

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