The Outer-Sphere Activated Complex

The discussion of experimental methods which opens this section applies equally well to the next two sections. The development of new methods of measurements of rates has been an important part of the progress in this field. The majority of the kinetic work done has been for systems in which there is no net chemical change. The reasons for this choice of problem are understandable they arise in part from the desire to have the chemistry simple and partly from the novelty and excitement of using isotopic tracers to measure rates of reactions, which until recently could not be measured by any other means. In fact, when the significance of isotopic methods in this field is appraised, it becomes obvious that of principal importance is the fact that the new methods attracted many capable 

A very ingenious method which in essence satisfies the conditions of no net change, and which can be applied to ions that exist in d-l forms, was introduced by Dwyer and Gyarfas 36). This method exploits the change in the rotatory power of a system in which a net change of the following kind is taking place  

It is regrettable that with all the activity that has developed in measuring rates of virtual changes there has been so little acceleration of the work for systems involving net chemical changes. Many of the important questions of mechanism which are posed can be answered as well by the study of orthodox reactions as they can by that of more exotic ones and, in most instances, by the expenditure of much less effort. The techniques which have been used are conventional, but for many of the important reactions further development of methods for the measurements of rates of rapid reactions is called for. Many of the systems of interest here involve intensely colored ions, so that the flow spectrophotometric method can often be applied. 

Among the reagents for which electron transfer can take place without net rearrangement of the coordination sphere are the following MnO  

The system among those of present interest which has been most thoroughly studied is exchange between Mn04 and Mn04=. A number of workers 21,61, 77,115) have done experiments with this reaction, but the most successful and complete study is that of Sheppard and Wahl 115). The rate of the reaction has been proved to be first order in each of the reactants, as was tacitly assumed in the earlier discussion of reactions of this class. The specific rate at 0° is reported as 710 M sec, E as 

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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... 

Another interesting point is the relative rates of the reactions of the azido and thiocyanatopentaammines. The relative rates of these two reactants with iron (I I) ion are similar to those with chromium (I I), that is, the azide is four to five powers of ten more rapid than is the thiocyanato. I am suggesting that this might be a criterion for inner sphere activated complex as opposed to an outer sphere complex. With trisdipyridylchromium(II) ion, which must react via an outer sphere process, the azido and thiocyanato rates are relatively comparable, and the same also for vanadium (I I) ion which also probably procedes via an outer sphere activated complex. 

Other reductions of 2,4,6-triphenylpyrylium ion to give 29 have been examined. Tetramethyl-p-phenylene diamine (TMPD) transfers one electron and ESR spectroscopy shows 29 and TMPD" " to result. Chromium(II) ion was shown to reduce 2,4,6-triphenylpyrylium ion and other related cations. A comparison of chemical reactivity with reduction potential suggested an outer-sphere activated complex. A scale of the relative stabilities of the various radicals was deduced. Since 29 results when 2,4,6-triphenyl-... 

A recent observation which may lead to an advance in understanding the operation of the bridged versus outer-sphere activated complex is this Cr(dip)s++ (I43) has been shown to react very rapidly with Co(III) complexes, including Co(NH8)e+++ V (dip)s++ reacts much less rapidly with the same Co (III) complexes. In these reactions we are almost certainly concerned with outer-sphere activated complexes. It will thus be possible to compare rates for the two types of mechanisms for a common group of oxidizing agents which can be formed in great variety. 

Radical intermediates have also been invoked in the mechanism of reduction of thiosulfate by Mn(VII). An outer-sphere activated complex is proposed in the one-electron transfer process leading to 8203" radical formation. Further rapid oxidation of the intermediate by MnO yields SOl. The reaction of S2O3" with [Mo(CN)8l is first order with respect to each reagent and independent of hydrogen ion concentration.Catalysis by metal ions is observed, however, indicative of a bridging role in the activated complex. 

Recent investigations suggest that the exchange of electrons may proceed by at least two different paths. One is called an electron transfer or outer-sphere activated complex mechanism. This envisions the oxidant and reductant coming together in an outer-sphere activated complex and thus permitting the transfer of an electron, Eq. (9). 

Now we can proceed to assemble the positive evidence for the path (I II -> IV, Fig. 7). Once the outer sphere complex, (II), is formed, all replacements of water should occur at the same rate, k - lO- If the ion pairing constant Ka is known, or a limiting rate of anion entry corresponding to saturation of the association is observable, the rates of conversion of (II) into (IV) may be compared for various X. All should be equal to / -h20 if the activation mode is d, but they will not equal the rate of water exchange which was identified with on the D path. The reason is that species (II) has a number of solvent molecules in its... 

The oxidation of thiols in the form of L-cysteine, penicillamine, and thioglycollic acid by [Mo(CN)g] in aqueous acidic solution also formed disulfides as final products 111). The reactions show a second-order substrate dependence, and the rates are found to decrease with increasing hydrogen ion concentration. This is attributed to the deprotonation of the —SH and —COOH groups in these thiols prior to electron transfer. The reactions are interpreted in terms of outer-sphere activation. An explanation for the second-order dependence on thiol concentration involves ion association between the cyano complex and a protonated form of the thiol, followed by reaction of this complex with a second thiol molecule. 

The net reaction for the reduction of Pu(VI) to Pu(V) by Fe(II) is quite simple in spite of this a complicated three-term hydrogen ion dependence was found (56). A mechanism which involves both outer-sphere and inner-sphere activated complexes is favored. The inner-sphere complexes are supported by evidence for consecutive reactions and a binuclear intermediate. 

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