Outer-sphere activated complex

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-... 

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... 

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). 

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. 

In triple layer approximations the location of each adsorbate with respect to the surface must be specified. Protons and all ions assumed bound as inner-sphere complexes (specifically or chemically adsorbed species) are assumed to lose part of their hydration sheaths, bonding directly to sites in the surface itself. Adsorbates assumed to remain hydrated, forming outer-sphere surface complexes, are assigned to the OHP. In the intrinsic equilibrium constants for adsorption reactions, K ", the activities of ions transferred from solution to the surface are corrected for the electrical potential they experience, % or (27). 

While lanthanide phosphate and carbonate stability constants increase substantially between La and Lu, the complexation behavior of lanthanides with sulfate changes very little across the lanthanide series. This difference in complexation constant trends is consistent with inner sphere (COj ) versus outer sphere (SO ) complexation behavior (Byrne and Li 1995). Stability constants for lanthanide sulfate complexes at 25 C and zero ionic strength could be well represented as logso4l8i(M) = 3.60d=0.08. The recommended stability constants (25 C, 0.7 mol kg ionic strength) shown in table 5 are based upon the works of Spedding and Jaffe (1954) and Powell (1974). Following the activity coefficient estimates of Millero and Schreiber (1982) and Cantrell and Byrne (1987a), lanthanide sulfate stability constants, expressed in terms of free-ion concentrations, were calculated as log 504 1 = log SO4/3 - 1.67. 

Differentiation between inner- and outer-sphere complexes may be possible on the basis of determination of activation volumes of dediazoniations catalyzed by various metal complexes, similar to the differentiation between heterolytic and homolytic dediazoniations in DMSO made by Kuokkanen, 1989 (see Sec. 8.7). If outer-sphere complexes are involved in a dediazoniation, larger (positive) volumes of activation are expected than those for the comparable reactions with inner-sphere complexes. Such investigations have not been made, however, so far as we are aware. 

Utilization of the Pfeiffer effect and outer-sphere complexation for the prediction of absolute configurations of optically active metal complexes. S. Kirschner and I. Bakkar, Coord. Chem. Rev., 1982,43, 325-335 (27). 

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... 

Reductions of various Co(ni) complexes by Fe(II) have been studied under high pressures . The motivation for performing such experiments resides in the possibility that the volume of activation (AF ), like the entropy of activation, might be a criterion for distinguishing between inner- and outer-sphere reactions. For reactions of the type... 

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