Electron transfer mixed valence complexes

Walker G 0, Barbara P F, Doom S K, Dong Y and Hupp J T 1991 Ultrafast measurements on direct photoinduced electron transfer in a mixed-valence complex J. Rhys. Chem. 95 5712-15... 

One final point should be noted. Theoretical discussions of electron transfer processes have focused almost entirely on outer-sphere processes. When we have an inner-sphere mechanism, or sufficient electronic interaction in a dynamically trapped mixed-valence complex to produce a large separation between upper and lower potential surfaces, the usual weak-interaction approach has to be abandoned. Thus a detailed knowledge of a potential surface which is not describable as an intersection surface of perturbed harmonic surfaces, for example, is required. For this purpose, detailed calculations will be required. The theory of these processes will be linked more... 

Electronic and vibrational spectroscopy continues to be important in the characterization of iron complexes of all descriptions. Charge-transfer spectra, particularly of solvatochromic ternary diimine-cyanide complexes, can be useful indicators of solvation, while IR and Raman spectra of certain mixed valence complexes have contributed to the investigation of intramolecular electron transfer. Assignments of metal-ligand vibrations in the far IR for the complexes [Fe(8)3] " " were established by means of Fe/ Fe isotopic substitution. " A review of pressure effects on electronic spectra of coordination complexes includes much information about apparatus and methods and about theoretical aspects, though rather little about specific iron complexes. ... 

The low effectiveness of the [Rh2"( Jt-OAc)4] system [31] in the oxygenation of alkenes has been attributed to its high oxidation potential to form the mixed-valence complex [Rh2" " ( Ji-OAc)4] on the basis of a relationship between the abihty of the complexes to transfer one electron and the effectiveness of the catalysts in allyhc oxidation, suggested by Kochi [33]. 

Fig. 13.13 Poienual energy diagram for a photochemicaDy induced homonudeor electron transfer in a mixed valence complex (R = reactants ...

Indirect evidence concerning intramolecular electron transfer has also been obtained by the observation of low-energy charge transfer absorption bands in mixed-valence complexes (reaction 8)14 even for outer-sphere electron transfer within ion pairs (reaction 9).15 The theoretical work of Hush makes it possible to use the energies and integrated intensities of such bands to estimate rates of intramolecular electron transfer.16... 

The results in Table III show that there is a rough parallel between the rates of net electron transfer from Ru(II) to Co (III) and the intensity of the intervalence band observed for the Ru(II)-Ru(III) mixed valence complex. Any such parallelism—more exactly a parallelism between AS+ and the intensities—would imply that a nonadiabatic factor does affect the rates of electron transfer. Unfortunately, it is not possible to examine this relationship in the sensitive region where the reactions are strongly nonadiabatic. The nir bands are very broad, and is difficult to measure extinction coefficients that are less than about 5 M"1 cm-1. 

An important application of the work on mixed valence complexes in which the metal ions are weakly but not too weakly coupled is in resolving the Franck-Condon barrier to electron transfer into the separate contributions from the inner-sphere and the solvent. Some work directed to this goal has been reported (62, 63). Because of the nature of the dependence of the solvent barrier on distance, those systems in which the separation of the metal centers is large will be particularly instructive. When this separation is sufficiently large, it may be possible to estimate the radius of the dielectrically saturated region about each ion so as to compare it with the radius of the coordination sphere itself. 

The intellectual push to study mixed-valence complexes was provided by the publication in 1967 of two review articles, by Allen and Hush (2) and Robin and Day (3), on the physical properties of mixed-valence systems. These were followed by Hush s publication (4) of his theoretical model of intervalence transitions, which provided a link between the properties of mixed-valence complexes in solution and the Marcus theory of intermolecular electron transfer (5, 6). The review by Robin and Day classified mixed-valence complexes into three types class I,... 

For a series of similar class II mixed-valence complexes that differ only in the nature of the bridging ligand, MMCT band energy will increase with increasing distance between the metal ions. This dependence has its origin in the magnitude of x as described by Eq. (19) and has been the subject of study (136,137). The relationship between Eop and the barrier to thermal electron transfer [Eq. (1)] was nicely demonstrated by a correlation between electron-transfer rate constants and intervalence-transfer energies (138) and is shown in Fig. 12. 

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