Bridging activation

Mechanisms of electron transfer reactions the bridge activated complex. A. Haim, Prog. Inorg. Chem., 1983, 30, 273-358 (196). 

Suggestions that the sulphate catalysed paths may involve a mechanism with a sulphate-bridged activated complex, as opposed to a hydrogen-atom transfer mechanism, have been made. ... 

In nitrate media ( 6 Af), fluoride ion has a catalytic effect on the exchange reaction between Ce(IV) and Ce(III). Hornig and Libby have made a detailed study of this effect, over the range of added KF, 0 to 8.4 x 10 M, and have concluded that a pathway involving a monofluoro complex occurs, possibly involving a fluoride-bridged activated complex. 

Traditionally, electron transfer processes in solution and at surfaces have been classified into outer-sphere and inner-sphere mechanisms (1). However, the experimental basis for the quantitative distinction between these mechanisms is not completely clear, especially when electron transfer is not accompanied by either atom or ligand transfer (i.e., the bridged activated complex). We wish to describe how the advantage of using organometals and alkyl radicals as electron donors accrues from the wide structural variations in their donor abilities and steric properties which can be achieved as a result of branching the alkyl moiety at either the a- or g-carbon centers. 

Accordingly, evaluation of k from line-width measurements gives a bimolecular rate constant of 0.5 X 1081 sec-1 M-1. This is one of the fastest electron exchange reactions studied in aqueous solution. Transfer rate here may not describe direct electron transfer between Cu1 and Cu11 Stranks has suggested (129) that the rate may refer to transfer within a bridged activated complex, e.g.,... 

The formation of a bridged, activated intermediate such as V is a well known hypothesis for explanation of many electron exchange and oxidation-reduction reactions (14). The two-electron reduction of niobium by pyridine makes this step necessary, since niobium (III) must be formed only as an intermediate. No evidence was found for the presence of niobium (III) in the final products. If... 

A second example when we compare the 7V s of a polyamide, e.g. PA-6.6 (260 °C) and polyethylene (135 °C), then we might, superficially, conclude that the higher value for PA is caused by a higher AH as a result of the strong hydrogen bridges active in PA ... 

During electron transfer reactions, the coordination spheres of the metal ions remain intact. By contrast, ligand transfer reactions proceed via a bridged activated complex in which the two metal ions are connected by a common bridging ligand. In the examples above, replacement by chloride of only one of the six ammonia ligands bound to cobalt accelerates the rate by a factor of over 109. 

Rapid exchange reactions between Mn04 and Mn04- have been studied, and cation effects were interpreted in terms of an outer-sphere bridged activated complex. 

Haiduc, lonel, see Tiekink, Edward, R. T., Haight, G. P., Jr., see Beattie, J. K. Haim, Albert. Mechanisms of Electron Transfer Reactions The Bridged Activated Complex 30 273... 

The encounter complexes exhibit high degrees of charge-transfer [20, 91], and on the basis of absorption and emission data electronic coupling matrix elements for similar complexes (exciplexes) have been determined [205] which are comparable to those of mixed-valence metal complexes commonly used as prototypical models for the bridged-activated complex in inner-sphere electron transfers [2, 26, 197]. Accordingly, we ascribe the unusually high rate constants, their temperature-independence, and their on-Marcus behavior to an inner-sphere electron transfer process [31]. 

The two types of activated complex that have been discussed present the gross geometrical features of two types which have been experimentally delineated however, the types that are observed do not necessarily present the other features which were referred to in the cases chosen for the qualitative theoretical discussion. In the next two sections the experimental evidence on the nature of outer sphere and bridged activated complexes is discussed (the terms have no connotations other than the geometry of the activated complexes implied by them), and the general observations relating to the kinetic behavior of the corresponding systems will be outlined. 

For a large number of reactions of Cr(III) complexes with Cr++, a bridged activated complex is obviously also involved. Among these is a reaction of almost classical interest the catalysis by Cr++ of the dissolution of anhydrous CrCls (f). The product of the reaction has been shown to be CrCl++ [rather than Cr(OH2)e+ + as would be expected for ordinary dilute solutions if complete equilibrium were rapidly established], and the Cl retained has been proved not to have passed through the solution (129). The reaction can be formulated as... 

An important comparison is that of the rate of reaction of Cr+ + with (NH3)eCo+ + + and with (NH8)sCoOH2+ ++. For the completely am-moniated species, the redox reaction is very slow, slower by at least a factor of 100 than for the aquo ion. The difference in rate can be ascribed to the availability of an electron pair when H2O is coordinated to a central ion all electron pairs are occupied for a coordinated NH3. The mechanism by which the hexammino ion is reduced is not known since the bridged activated complex has been made difiicult of access, electron transfer may in fact take place through the coordination spheres of the... 

Some general observations on the energies and entropies of activation of redox reactions which proceed by bridged activated complexes are in order. These quantities, even for the few systems for which they have been determined, cover the range 4 to 14 kcal and —20 to —45 e.u. respectively. The ranges overlap with those for the outer-sphere activated complexes and, except possibly in extreme cases, it is not safe to use the magnitude of these quantities as diagnostic of mechanism. The comparison of AS for the process... 

When an activation energy as small as 4 kcal is in question for the bridged activated complex, we face the difficulty that the activation energy for substitution on the Cr++ probably exceeds this value. A mechanism in which the reaction occurs in a single step,... 

---