Transition metal electron transfer mechanism

Two types of electron transfer mechanisms are defined for transition metal species. Outer-sphere electron transfer occurs when the outer, or solvent, coordination spheres of the metal centers is involved in transferring electrons. No reorganization of the inner coordination sphere of either reactant takes place during electron transfer. A reaction example is depicted in equation 1.27 ... 

Most PET fluorescent sensors for cations are based on the principle displayed in Figure 10.7, but other photoinduced electron transfer mechanisms can take place with transition metal ions (Fabbrizzi et al., 1996 Bergonzi et al., 1998). In fact, 3d metals exhibit redox activity and electron transfer can occur from the fluorophore... 

All these observations point to the occurrence of a 8 2 rather than an outer sphere, dissociative electron-transfer mechanism in cases where steric constraints at the carbon or metal reacting centres are not too severe. It is, however, worth examining two other mechanistic possibilities. One of these is an electrocatalytic process of the Sg -type that would involve the following reaction sequence. If, in the reaction of the electron donor (nucleophile), the bonded interactions in the transition state are vanishingly small, the alkyl radical is formed together with the oxidized form of the electron donor, D . Cage coupling (144) may then occur, if their mutual affinity is... 

Reaction (63) is an example of 0 acting as an oxidant and it probably proceeds via an inner-sphere electron transfer mechanism in which incompletely coordinated Cu binds O2 prior to electron transfer [87]. HO2 and 0 also react readily with a number of other transition metal ions, either by electron transfer or through the formation of a complex [83], for example ... 

Cytochrome c, a small heme protein (mol wt 12,400) is an important member of the mitochondrial respiratory chain. In this chain it assists in the transport of electrons from organic substrates to oxygen. In the course of this electron transport the iron atom of the cytochrome is alternately oxidized and reduced. Oxidation-reduction reactions are thus intimately related to the function of cytochrome c, and its electron transfer reactions have therefore been extensively studied. The reagents used to probe its redox activity range from hydrated electrons (I, 2, 3) and hydrogen atoms (4) to the complicated oxidase (5, 6, 7, 8) and reductase (9, 10, 11) systems. This chapter is concerned with the reactions of cytochrome c with transition metal complexes and metalloproteins and with the electron transfer mechanisms implicated by these studies. 

The relatively high reactivity of the allyl-NF complexes toward aryl halides, especially bromides, is unusual among traditional organic mechanisms and represented one of the first observations of this special feature in transition metal reactions. An Sn2 mechanism for activation of the halide is clearly ruled out, and the reactivity correlates better with an electron-transfer mechanism. The rates are roughly correlated with ease of reduction of the organic halide. This also is consistent with the low reactivity of / -toluenesulfonate esters, compared to bromides. ... 

The electron-transfer mechanism supposes that the unpaired electrons in the G -band of a transition metal will pair up with tt electrons of the graphite in contact with the metal. As a result, the number of mobile n electrons in the graphite decreases. This, in turn, will weaken the average strength of the bonds between peripheral carbon atoms and carbon-oxygen complexes that are intermediates in the gasification process. Hence, the liberation of these to the gas phase will be facilitated and gasification enhanced. 

Reactions of free radicals with molecules (or ions) can occur via an addition reaction, for instance the addition of a C-centered radical to oxygen (4.2), hydrogen atom abstraction (4.3), or electron transfer mechanism (e.g., oxidation of CO " by SO" ). Since the total number of electrons is odd, one of the products is a new radical (e.g., (4.2) and (4.3)), except for the reactions with transition metal ions, such as the oxidation of superoxide radicals by Fe3+ ions ... 

Post-heating of a shaped polyethylene which induces its crosslinking may sometimes lead to an undesired deformation of the product. To overcome this difficulty, two-component redox-initiating systems producing free radicals at lower temperatures have been designed [108]. The reaction system involved cumyl hydroperoxide and a transition metal ion, whose higher and lower oxidation states differ by one electron. The decomposition of hydroperoxide proceeds by an electron transfer mechanism an electron is transferred from the metallic ion (e.g. Co ) to the peroxidic bond which splits into two fragments. 

A number of reports on transition metal-free formations of benzo [c] cinnolines emerged in 2014. Sythana and collaborators prepared benzo [c] cinnoline N-oxides via a single-electron transfer mechanism (Scheme 11) (14EJ0311). In the presence of potassium tert.-butoxide and an aptotic nonpolar solvent benzo [c] cinnoline N-oxides 18 were obtained in moderate-to-high yields from the reaction of substituted anilines with nitrobenzenes. Both electron-donating substituents and electron-withdrawing halo substituents were well tolerated. When the halo substituents were in the... 

Obviously a discussion of the oxidative addition reactions which proceed by an electron-transfer mechanism is outside the scope of this review for a thorough appraisal of this area the reviews by Halpern (1970), Lappert and Lednor (1976), Stille and Lau (1977), Deeming (1972, 1974), Kochi (1978), together with the recent article by Hill and Puddephatt (1985) and references therein, should be consulted. For the remainder of this chapter we will be concerned entirely with systems in which the nucleophilicity of the metal complex has been either unambiguously demonstrated, or at least strongly implicated. It is important to be conscious of the spectrum of transition-state structures which could exist in the interaction between a metal centre and a molecule of RX as shown in Fig. 1. 

The related electron transfer mechanism for the reduction to hydrogen peroxide is shown in Fig. 16.12. The authors concluded that there are no evidences for a direct reduction to water, but that the integration of transition metals might enhance the catalytic interaction to enable the four-electron transfer process [135],... 

The simplest electrochemical reactions, which can be foxmd among the different kinds of electrode processes, are those where electrons are exchanged across the interface by flipping oxidation states of transition metal ions in the electrolyte adjacent to the electrode surface (Bamford Compton, 1986), i.e. an ET (electron transfer) mechanism. The electrode acts as the source or sink of electrons for the redox reaction and is supposed to be inert. The reduction of ferricyanide to ferrocyanide (Angell Dickinson, 1972 Bamford Compton, 1986 Bruce et al., 1994 Iwasita et al., 1983) is an example of such a mechanism ... 

Hydroperoxides are more widely used as initiators in low temperature appHcations (at or below room temperature) where transition-metal (M) salts are employed as activators. The activation reaction involves electron-transfer (redox) mechanisms ... 

Chemical, or abiotic, transformations are an important fate of many pesticides. Such transformations are ubiquitous, occurring in either aqueous solution or sorbed to surfaces. Rates can vary dramatically depending on the reaction mechanism, chemical stmcture, and relative concentrations of such catalysts as protons, hydroxyl ions, transition metals, and clay particles. Chemical transformations can be genetically classified as hydrolytic, photolytic, or redox reactions (transfer of electrons). 

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