Self exchange rates, table

Table 5.6 Calculated Values for the Self-Exchange Rate Constant for Ru(H20)6+ + using (5.35) and Data for a Number of Cross-Reactions (from Ref. 43)...

Table 7.8 Kinetics and thermodynamics of protonated amine encapsulation in 1. The self-exchange rates ( 277) ofthe protonated guests were measured at 277 K. /< eff(298) is the binding constant of the encapsulated protonated amine and has an estimated uncertainty of 10%.

In Table 1 are summarized representative examples of self-exchange rate constant data for a variety of different types of redox couples based on metal complexes, organometallic compounds, organics and clusters. Where available the results of temperature dependence studies are also cited. For convenience, data obtained from temperature dependence studies are presented as enthalpies and entropies of activation as calculated from the reaction rate theory expression in equation (14). 

Table 1 Representative Self-exchange Rate Constants3...

A point of note in the data in Table 1 is the extraordinary range in electron-transfer reactivity that can exist even for outer-sphere reactions among what appear to be closely related reactions. For example, the self-exchange rate constants for Co(NH3)63+/2+ and Ru(bipy)33+/2+ differ in magnitude by 1014. 

The agreement between the calculated and observed results in Table 2 is quite remarkable and provides strong confirmatory evidence for the validity of the overall theoretical approach. In addition, the quantitative partitioning of the rate constant into its various components is most revealing as to the factors that dictate rates of electron transfer and leads to a basis for discussing the variations in self-exchange rate constants in Table 1. 

An important feature to emerge from the comparisons in Table 2 is that variations in the electronic coupling term play a relatively small role in dictating the magnitudes of self-exchange rate constants for outer-sphere reactions, at least for transition metal complexes. Even for reactions... 

From the experimental point of view, significant variations in kobs can be induced by changes in solvent and/or molecular size. For example, there are relatively small contributions to A for the self-exchange reactions for the Ru(NH3)63+/2+and Ru(bipy)33+/2+couples in Table 1 and the effects of the differences in molecular radii on KA and Ao are sufficient to account for the difference in self-exchange rate constants of 106. 

Nonetheless, in Table 4 are summarized the results of a series of rate constant comparisons between calculated values using optical absorption data and experimental values.83 The experimental data are derived from self-exchange rate constants for couples which are structurally related to the mixed-valence dimers shown in the table. The experimental values cited are calculated values for electron transfer within the association complex between reactants as estimated from k kQ >JKA. values were calculated using equation (33) and it was assumed in the calculations that vet = 5 x 1012 s-1. Calculations of this kind have been extended to unsymmetrical dimers like (NHJ)5RuIII(pz)RuIICl(bipy)24+ 83 and even to outer-sphere ion-pairs like (4).88... 

Table 4 Comparisons between Self-exchange Rate Constants and Intramolecular Rale Constants Calculated from IT Band...

By leaving all other terms constant one cannot expect accurate predictions of the self-exchange rates. However, the surprisingly good results (Table 10.2) indicate that the relative strain in the ground and transition states of the encounter complex is an important driving force for the electron transfer reactivity. 

Table 10.2. Observed and calculated electron self-exchange rates of hexaamine cobalt(III/II) complexes11321.

What first strikes the eye in Table 12 is the variation in A, which is to be expected from an inspection of the individual values of A for self-exchange reactions that are listed in Table 6. No doubt the considerable scatter of data points in the Marcus plots reflects to a large extent this variation in X. It is therefore important to study series of closely similar compounds to test the theory, as was indeed pointed out very early by Marcus (1964). Preferably, one should work with compounds of known X, determined by independent measurements of self-exchange rate constants. 

In some recent experiments (14) on rates of oxidation by Fe3+ of a number of related Ru(II) species, the rates of the self-exchange reactions for Ru(III)-Ru(II) couples and the equilibrium data were measured as much as possible under uniform conditions. In determining self-exchange rates, a series of reactions of the type Ru(NH3)5L2+ + Ru(NH3)5L 3+ were studied in which L and L are pyridines which differ in one substituent in the 3 or 4 position. No rate differences ascribable to differences in L and L were observed, apart from the effect on driving force. The result of these studies are summarized in Table I. 

Table IV. Electron Transfer Cross-Reaction and Self-Exchange Rate Constants for Blue Copper Proteins (25°, /aO.IM, pH 7)a...

Fig. 12. Plot of number of amine protons versus log of the self-exchange rate constant (M s ) for cobalt hexaamine complexes at 25°C. No correction has been made for ionic strength differences. The data include some nonhomoleptic complexes. (1) [CoCNHslg], (2) [Co(en)3]3+ 2+, (3) [Co(chxn) ] + 2+ (4) [Co(tmen) ]3+ 2+, (5) [Co(dien)"]"+ 2+, (6) [Co(pet) P+ 2+, (7) [Co(linpen)P" 2 + (g) lCo(medien)(9) [Co(tacn)(dien)]3+ 2+, (10) [Co(tacn) (pet) (11) [Co(tacn) (etdien) (12) [Co(tacn) (budien) p+ 2+ 3 [Co(tacn)(medien)P, (14) [Co(diAmsar)] , (15) [Co(taptacn)P, (16) [Co(metacn) ] 2+, (17) [Co(diAmsar)P 2+, (18) [Co(sar)(19) [Co(sep)P 2, (20) [Co(dtne)] , (21) [Co(Amsartacn)], (22) [Co(Amsartacn)] , (23) [Co-(diAmchxnsar)] , (24) [Co(diAmchxnsar)] . The data for homoleptic complexes are taken from Table IV the other data are from reference U02). The line was calculated without the data for the sep and sar derivative cages and the [Coftmenls] couple.

Table 1. Rate constants for electron self-exchange rates determined for different Co /Co systems in water at 25 C.

Table I. Values of Reduction-Potential and Self-Exchange Rate Constants Several Reductants FOR...

Reactivity patterns in widely varying oxidants are seldom considered, the reduc-tant patterns being more often compared. Such studies can be approached in the same way as that of reductants, but because the [CoCNHjjjX]"" oxidants are so common, there may be more difficulties in determining both the self-exchange rate and reduction potential. Table 1 lists values of and for several oxidants, as well as the calculated oxidation factors (OF) [using (f) in 12.2.5.1.1]. These OF values can be corrected to give effective oxidation (actors, but because fewer reversals of trends appear, the effective oxidation factors are not included here. The OF values suggest a reactivity pattern with a reductant of = 0.3 of [Ru(bipy)3] > [Fe(l,10-phen)3 + > [IrBr ] ",... 

Table 1. Redox Potentials and Self-exchange Rates of Ground- and Excited-State...

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