The Bridged Activated Complex

The term bridged will be used to imply a configuration in which an atom or group of atoms is part of the coordination sphere of both cation partners in the redox reaction. The characteristic feature is that the coordination sphere of at least one of the partners has been entered, so that new bonds are established in making the activated complex. It is by no means necessary that this geometry correspond to the strong overlap (8S) case, although, as will be adduced from the evidence for the cases 

Complex ions of the series (NHs)6Co L can be formed in great variety, and these are useful for the present purposes in providing a survey of groups which will act in the same capacity as Cl. Efficient transfer from (NHa) sGo L to Cr++ is observed also for F, Br, I, S04= (129), Ns, CNS , carboxylic acids, (125), P04=, OH (92). Particularly 

The kinetic data are summarized in Table IV. The specific rate k2 which is the coefficient for the term (R0OH++) (Cr++) is obviously equal to fc27 K where K is the dissociation constant of RoOH2 +. The value of K has been.determined (16) as 1.2 X 10 at 25° and = 1.00 the associated value of Aff is 10 kcal. The value of is a revision of that reported earlier (92), but the new value of 2 agrees well with the earlier one. 

The tracer work on oxygen atom transfer has already been referred 

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 

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

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. 

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]. 

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... 

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,... 

Haim, Albert, Mechanisms of Electron Transfer Reactions The Bridged Activated Complex. 30 273... 

Step 2 The electron transfer from Cr (II) to Co (III) in the bridged activated complex occurs through the chloro bridge. This results into oxidation of Cr (II) to Cr (III) and reduction of Co (III) to Co (II). Step 3 The Cr (III) attracts the Cl ion more strongly, as compared to Co (II). Due to this, the chloro ligand becomes a part of the chromium complex in the final product. 

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. 

The nature of the bridging thiolate ligands or the replacement of a terminal chloride ligand by water did not have much effect on the catalytic activity, complexes 105b-d and 106a,b being also operative in these transformations. In contrast, conventional monometallic ruthenium derivatives, as well as diruthenium complexes having no Ru-Ru bond, did not work at all. 

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.,... 

In the catalytically active complex 4-Ba the negative poles and the polyether bridge act as working units that perform cooperatively in providing the driving force for the formation of the complex itself, whereas the metal ion serves as an electrophilic catalyst both in the acylation and deacylation steps. The crucial importance of the polyether bridge is demonstrated by the disappearance of any catalytic activity upon replacement by two methoxy groups. 

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. 

Most of these reactions are reversible under either thermal or photochemical conditions. Interestingly, the bridging alkylidene complex (32) is redox active. 

The detection of [PhIO(salen)Mn-0-Mn(salen)OIPh]2+ in the ESMS experiments was the first direct observation of the conproportionation of Mnm and Mnv-oxo species as the mechanism for parking the catalytically active complex in a more persistent form, the mechanism postulated earlier by Kochi et al. [102]. The microscopic reverse process, the disproportionation of the p-oxo bridged dinuclear complex, would lead to the release of [0=Mnv(salen)]+. This... 

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