Precursor complex formation constant, Marcus theory

The outer sphere character of these reactions has encouraged some workers to apply Marcus theory to the rate constants obtained " . Given the uncertainty in the values of the electrode potentials and the considerable electrostatic work function involved in the formation of the precursor complex, the significance of the intrinsic rate parameters obtained is not clear. 

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... 

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