Bridged activated complex mechanism

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. ... 

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... 

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,... 

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

Tanbe H. and Myers H. (1954), Evidence for a bridged activated complex for electron transfer mechanisms , J. Chem. Soc. 76, 2103-2111. 

A similar mechanism is proposed for the reduction of monohalogenacetic acids by Cr +. The halogen is transferred to the reductant in a bridged activated complex to give CrX + and the carbon-centred radical CHaCOaH. Subsequent reaction of this species with Cr + results in [CrCHaCOaH] +. The reaction rates decrease in the order X=I> Br > Cl and may reflect the strength of the C—X bond. 

Whenever there is an opportunity for bridge formation then the mechanism is one called atom transfer or bridged activated complex. This mechanism differs from the previous one mentioned in that the oxidant and reductant come together in a more intimate fashion, with a bridging ligand being directly coordinated to both the oxidant and the reductant, Eq. (10). 

A mechanism similar to that shown in Scheme 16.2 has also been proposed to rationalize the arene hydrogenation activity of the triply-bridged dirhodium complex L2HRh(/7-H3)RhL2 (L=P(OiPr)3) [14]. 

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