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Inner-sphere reactions defined

Electrode reactions are inner-sphere reactions because they involve adsorption on electrode surfaces. The electrode can act as an electron source (cathode) or an electron sink (anode). A complete electrochemical cell consists of two electrode reactions. Reactants are oxidized at the anode and reduced at the cathode. Each individual reaction is called a half cell reaction. The driving force for electron transfer across an electrochemical cell is the Gibbs free energy difference between the two half cell reactions. The Gibbs free energy difference is defined below in terms of electrode potential,... [Pg.311]

This chapter mainly focuses on the reactivity of 02 and its partially reduced forms. Over the past 5 years, oxygen isotope fractionation has been applied to a number of mechanistic problems. The experimental and computational methods developed to examine the relevant oxidation/reduction reactions are initially discussed. The use of oxygen equilibrium isotope effects as structural probes of transition metal 02 adducts will then be presented followed by a discussion of density function theory (DFT) calculations, which have been vital to their interpretation. Following this, studies of kinetic isotope effects upon defined outer-sphere and inner-sphere reactions will be described in the context of an electron transfer theory framework. The final sections will concentrate on implications for the reaction mechanisms of metalloenzymes that react with 02, 02 -, and H202 in order to illustrate the generality of the competitive isotope fractionation method. [Pg.426]

Reactions that involve O2 as the reactant or the product may occur by an inner- or outer-sphere pathway when the redox partner is a transition metal. The same is true of reactions that consume or produce (V-. The inner-sphere reaction is defined by the presence of a bond to 02 in the transition state. Different types of inner-sphere reactions are possible including those that form covalent intermediates and those that do not. The outer-sphere reaction simply converts O2 to C>2, or vice versa, in the absence of covalent bonding. The free energy barrier in the reaction is expected to arise from the reorganization needed to accommodate electron transfer and the redistribution of charge. An additional contribution derives from lengthening or contracting the 0—0 bond. [Pg.438]

Another classification of C-H activation methods is as inner-sphere and outer-sphere mechanisms. Inner-sphere mechanisms can be defined as those that involve the formation of a carbon-metal bond from a C-H bond, while outer-sphere mechanisms involve the cleavage of a C-H bond by a metal-containing species to generate a reactive intermediate, but without a metal-carbon bond. A disadvantage of this classification is that it assumes that the mechanism is known The reactions discussed in this chapter would be considered inner-sphere. Reactions such as the Fenton reaction would be considered outer-sphere. A grey area is likely to exist between the two mechanisms. Another disadvantage of this classification is that the term inner-sphere mechanism tells us nothing about the mechanism beyond the formation of a metal-carbon bond ... [Pg.91]

In this chapter, the reactions of metal ions in a high oxidation state with inorganic and organic substrates are discussed. Such investigations have provided much mechanistic data and an increasingly important aspect is the evidence for inner-sphere reactions with the formation of metal-ion complex intermediates. The question of replacement as a prerequisite to redox reactions has been discussed and the role of the intermediate defined. It is of interest to note that identification of an intermediate does not necessarily infer its involvement in the rate-determining process, in that, for the reactions... [Pg.40]

In view of the discussion just previous, it is natural to inquire into the circumstances under which the investigation of precursor complexes might lead to an assignment of inner-sphere vs. outer-sphere mechanism. The issue is not independent of the previous discussion because the successor complex for the forward reaction is the precursor complex for the reverse. If the reaction mechanism has been defined for the forward direction, it is defined also for that portion of the reverse reaction which makes use of the same path. But in terms of the experimental criteria which are... [Pg.368]

As has already been acknowledged, classifying a reaction mechanism as inner sphere only advances us part way to the goal of defining the nature of the activated complex. In this section the most thoroughly studied reaction of the inner-sphere class is chosen for description and discussion so as to indicate some of the additional issues which can profitably be addressed. [Pg.371]

A large number of radical reactions proceed by redox mechanisms. These all require electron transfer (ET), often termed single electron transfer (SET), between two species and electrochemical methods are very useful to determine details of the reactions (see Chapter 6). We shall consider two examples here - reduction with samarium di-iodide (Sml2) and SRN1 (substitution, radical-nucleophilic, unimolecular) reactions. The SET steps can proceed by inner-sphere or outer-sphere mechanisms as defined in Marcus theory [19,20]. [Pg.284]

An interesting subset of the inner-sphere electron transfer reactions involves the irreversible formation of a stable metal-02 adduct as the product. A series of such reactions (Figure 9.4) has been investigated by reacting d8 and d10 organometallic complexes with O2.45 These reactions result in the formation of structurally defined side-on peroxide complexes. [Pg.439]

Variation in the metal surface composition is, then, generally expected to yield large variations in the observed rate constant for inner-sphere pathways since the reaction energetics will be sensitive to the chemical nature of the metal surface. For outer-sphere reactions, on the other hand, the rate constants are anticipated to be independent of the electrode material after correction for electrostatic work terms provided that adiabatic (or equally non-adiabatic) pathways are followed. Although a number of studies of the dependence of the rate constants for supposed outer-sphere reactions on the nature of the electrode material have been reported, relatively few refer to sufficiently well-defined conditions where double-layer corrections are small or can be applied with confidence [111-115]. Several of these studies indeed... [Pg.49]

A major application of eqn. (47) is to diagnose the presence of catalytic, presumably inner-sphere, electrochemical pathways. This utilizes the availability of a number of homogeneous redox couples, such as Ru(NH3)e+/2+ and Cr(bipyridine) +,2+ that must react via inner-sphere pathways since they lack the ability to coordinate to other species [5]. Provided that at least one of the electrochemical reactions also occurs via a well-defined outer-sphere pathway, the observation of markedly larger electrochemical rate constants for a reaction other than that expected from eqn. (47) indicates that the latter utilizes a more expeditious pathway. This procedure can be used not only to diagnose the presence of inner-sphere pathways, but also to evaluate the extent of inner-sphere electrocatalysis (Sect. 4.6) it enables reliable estimates to be made of the corresponding outer-sphere rate parameters [12a, 116, 120c]. [Pg.53]

Oxidants obtained directly from easily oxidized halides often react chemically with organic substrates the resulting adduct may then undergo a reaction that regenerates the halide ion. Indirect oxidations of this kind are essentially inner sphere processes, as previously defined. [Pg.1187]

In 1954, King and Taube published the 1980 Nobel Prize winning work that defined these two different types of electron transfer reactions. In an inner-sphere mechanism, the atoms undergoing redox form bonds to a common atom (or small group of atoms), which then serves as a bridge for electron transfer (ISPC = inner-sphere precursor complex and ket = electron transfer rate constant). [Pg.12]

The enantioselectivity associated with quaternary allylation is connected with scenario 5 above (one of the five points associated in the catalytic cycles shown by Schemes 12.10a and b where chirality could be induced), which is where enantioselection of one of two faces of the nucleophile (the enolate ion) occurs. Theoretical studies of the transformation using the PHOX ligand have shown support for an inner sphere mechanism, where nucleophilic attack of the enolate onto the rf-allyl ligand occurs from the Pd-bound enolate and not from an external nucleophile.74 These studies have not been able to definitively determine the step that defines the enantioselectivity of the reaction, and it is not clear how these results would carry over to reactions involving the Trost ligands. At this time, selection of which ligand one should use not only to induce enantioselectivity but also to predict the sense of absolute configuration of any asymmetric Tsuji-Trost allylation is mostly based on empirical results. Work continues on this... [Pg.566]


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See also in sourсe #XX -- [ Pg.2 , Pg.2 , Pg.2 , Pg.2 , Pg.2 , Pg.12 , Pg.12 ]




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