Outer-sphere reactions defined

Section II focuses on outer-sphere reactions of species that are stable in their adjacent oxidation states, which leads to a degree of confidence in the reaction mechanisms and the ability to define and measure the... 

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. 

Several distinctly different classifications can be established. The simplest of these concerns the spatial position of the reaction site in the interfacial region. So-called outer-sphere reaction pathways are defined as those... 

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... 

Outer-sphere reactions and ET within rigid complexes of well-defined geometry proceed without changes in the chemical structure, since bonds are neither formed nor broken. 

In the same way that we considered two limiting extremes for ligand substitution reactions, so may we distinguish two types of reaction pathway for electron transfer (or redox) reactions, as first put forth by Taube. For redox reactions, the distinction between the two mechanisms is more clearly defined, there being no continuum of reactions which follow pathways intermediate between the extremes. In one pathway, there is no covalently linked intermediate and the electron just hops from one center to the next. This is described as the outer-sphere mechanism (Fig. 9-4). 

Outer-sphere (OS) reaction rates and rate laws can be defined for solvolysis of a given complex. Complex formation is defined as the reverse reaction—that is, replacement of solvent (S) by another ligand (L )- Following the arguments of... 

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 ... 

Ion pairs are outer-sphere association complexes, which have to be clearly distinguished from the organometallic complexes discussed in Section 6. Ion pair formation appears to be much less important in biological membranes as compared with octanol, because the charge of the ions at the membrane interphase can be balanced by counter charge in the electrolyte in the adjacent aqueous phase. The reactions involved in ion pair formation are depicted in Figures 5b for acids and 5c for bases, and the equilibrium constant K ix is defined as follows ... 

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... 

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]. 

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. 

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