Ferrocene self-exchange

The value of ket > 6 x 108 cm8mol ls - - in the usual units for a bimolecular rate constant for homogeneous solution is > 6 x 105 M-ls l. The ferrocene self-exchange constant is 5 x 106 M-Ts-1 (29). Various cross reactions of substituted ferrocenes and ferricenium derivatives have bimolecular rate constants that exceed 10 M - -s l where the equilibrium constant exceeds unity (30). Further, in the cross reactions, the rate constants varied by almost two orders of magnitude for a change in driving force of -0.25 V (30). Thus, the data in Table II relating to the ferrocene-like molecules is reasonable. 

The mechanism of an oxidation-reduction reaction can be simple, as illustrated by the ferrocene-ferricenium self-exchange in equation (1) where only electron transfer need occur.1 In other cases the mechanistic demands imposed by the net reaction are far greater. An example is shown in reaction (2) where, in the net sense, two protons and two electrons must be transferred from isopropanol, which is the reducing agent, to the RuIV oxidizing agent.2... 

The efficiency of electron-transfer reduction of Cgo can be expressed by the selfexchange rates between Coo and the radical anion (Ceo ), which is the most fundamental property of electron-transfer reactions in solution. In fact, an electrochemical study on Ceo has indicated that the electron transfer of Ceo is fast, as one would expect for a large spherical reactant. This conclusion is based on the electroreduction kinetics of Ceo in a benzonitrile solution of tetrabutylammonium perchlorate at ultramicroelectrodes by applying the ac admittance technique [29]. The reported standard rate constant for the electroreduction of Ceo (0.3 cm s ) is comparable with that known for the ferricenium ion (0.2 cm s l) [22], whereas the self-exchange rate constant of ferrocene in acetonitrile is reported as 5.3 x 10 s , far smaller than the diffusion limit [30, 31]. 

Figure 4. The activation energy of ferrocenium-ferrocene (Cp2Pe+/°) self-exchange in acetone (e = 20.70) as the function of electrolyte concentration, (a and ) - experimental values for tetraethylammoniym perchlorate and tetraethylammoniym hexafluorophosphate correspondingly [42], (solid and dashed line)-theoretical prediction from the AMS A theory with parameters Rd = Rb. = L = 8 A, Ri = 6 A, and ft, = 8 A, respectively [41],...

The kinetic behavior of the reductions of several Cu(II)N2S2 complexes, containing thioether/pyridyl chelate ligands, by ferrocene and l,r-dimethyl-ferrocene in acetonitrile points to the formation of a precursor complex prior to electron transfer.The rate constant for the oxidation of (hydroxyethyl)-ferrocene by [2-pyridyl(methylbis(2-ethyl)thioethyl)amine]copper(II) yields a [Cu(pmas)] self-exchange rate constant of 47 M s from the Marcus theory relation.The addition of NJ increases the rate of oxidation (F" and I" have no effect) by shifting the reduction potential upon the formation of [Cu(pmas)N3] and Cu(pmas)(N3)2 (NJ displacement of a thioether sulfur occurs in the latter species). The application of the Marcus relationship to the reductions of the [l,8-bis(2-pyridyl)-3,6-dithiaoctane]copper(II) complex by a series of Ru(II) ammine and bipyridyl complexes in 50% aqueous CH3OH yields a self-exchange rate constant of 0.63 s for the [Cu(pdto)] couple. " From the rate... 

The electron self-exchange rate constants for several Fe(II)/Fe(III) porphyrin couples have been measured by H NMR line-broadening techniques in 5 1 acetone/water at -20 The relative rate constants for the [Fe(P)(l-MeIm)2] couples, P = octaethylporphyrin chlorin < isobacteriochlorin, have been attributed to differences in outer-sphere reorganization, related to the steric bulk. The rate-determining step in the metallopophyrin-catalyzed reductions of dioxygen by substituted ferrocenes is the electron transfer between the ferrocene and the metalloporphyrin (M = Fe, Co, and The Marcus relationship provides a... 

In a few cases, however, a more quantitative analysis can be undertaken whereby the desired solvent-dependent AG int values are obtained experimentally from optical ET reorganization energies for related binuclear systems. An example of this approach, involving self exchanges of metallocenium-metallocene (Cp2M ") redox couples in various polar Debye solvents is worth highlighting here since ET rate-solvent dependencies are observed that span the limits of nonadiabatic and adiabatic behavior [12]. For ferrocenium-ferrocene couples, the rate constants k gx (corrected for AG -solvent variations) are almost independent of the solvent dynamics, as... 

Ferrocene, [Fe(Cp)2], self-exchange has also been studied by nmr in a variety of solvents. The rates do not vary with solvent dielectric as predicted by Marcus ... 

Figure 16.4 Solvent dependence of the free energy of activation of self-exchanges as a function of the solvent optical and static dielectric constants. The free energies were obtained from the experimental rates using the frequency factor from transition-state theory, and are relative to those in acetonitrile. Tetracyanoethylene " squares [12] and solid line, calculated with eq. (16.13) using r = 6.7 A ferrocene circles [13] and dashed line, calculated with eq. (16.13) using r = 7.6 A sesquibicyclic hydrazine 22/u22 triangles [14].

The double-layer effects are significantly reduced when one of the partners in the redox couple is uncharged. This is the case of ferrocene, which was treated in detail for gas phase and homogenous solution self-exchanges. The standard rate constants for the electro-oxidation of ferrocene at a platinum electrode in acetonitrile range from 0.02 to 220 cm sec" [46]. Some of the observed scatter is related to differences in electrolyte nature and concentration, or in temperature, but also reflects the experimental errors of some of the techniques employed. Fawcett and OpaUo reviewed these data and recommended a... 

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