Electron transfer mixed valence ions

A theoretical formalism is available for understanding optical charge transfer processes in a variety of chemical systems (mixed-valence ions, donor-acceptor complexes, metal-ligand charge transfer chromophores, etc) where the extent of charge transfer is large and where electronic coupling between the electron donor and acceptor sites is relatively small. 

The relatively long lifetimes of the excited states of these complexes have made them particularly attractive in the study of electron and energy-transfer quenching of excited states through organic bridges. In addition, the more positive redox potentials of these ions, compared with their pentaammine counterparts, mean that the mixed-valence ions are not air sensitive, thus facilitating spectroscopic measurements. 

Optical electron transfer in mixed-valence ions 359... 

Models for the static spectral sensitization of coordination compounds are discussed. Among them, ion pairs with oppositely charged metal complexes, where intervalence charge-transfer interactions occur, appear to be promising exarrples. The spectra and the electron transfer kinetics,of mixed-valence ion pairs of ihe type M /fMCNjgJ4-, where M + = Fe3 Cu2 UO V02 are discussed in detail. 

Figure 1 Hush diagram for intervalence transfer within a class II mixed-valence ion. The dotted lines correspond to diabatic potential energy surfaces. The solid lines are adiabatic potential energy surfaces. Electron transfer can occur either optically (vertical transition with energy, Eop, equaling A) or thermally by moving along the lower adiabatic surface. In the diabatic limit, the barrier height for thermal electron...

Calculations of electron transfer rate constants from the energies of intervalence transfer (IT) bands in mixed-valence complexes have provided some interest (Table 1.1). The ion pair formed from paraquat (l,r-dimethyl-4,4 -bipyridine " ), and ferrocyanide, [PQ ", Fe(CN)6 ], shows an IT band from which the activation energy for thermal electron transfer within the ion pair can be derived (Figure 1.1) using Hush s theory to compare spectroscopic and kinetic data [equation (5)]. 

In the thermodynamically redox-stable resting state, CcOs all Cu ions are in the Cu state and all hemes are Fe . From this state, CcOs can be reduced by one to four electrons. One-electron reduced CcOs are aerobically stable with the electron delocalized over the Cua and heme a sites. The more reduced forms—mixed-valence (two-electron reduced), three-electron reduced, and fully (four-electron) reduced—bind O2 rapidly and reduce it to the redox level of oxide (—2 oxidation state) within <200 p-s [Wikstrom, 2004 Michel, 1999]. This rate is up to 100 times faster than the average rate of electron transfer through the mammalian respiratory chain under normal... 

In any mixed-valence compound, the first problem to be addressed is one of description. Is the compound localized or "delocalized In a localized description the stationary state quantum mechanical solution for the odd electron is oscillatory in nature and has the electron transferring back and forth between the metal ion sites. Spectroscopic data are available for... 

Methods discussed so far for determination of x assume that two ions are present and that electron transfer can be observed —as, e.g., in a mixed-valence system. What can be obtained for systems (e.g., Fe(III)(H20) /Fe(II)(H O) ) where this is not... 

In the fully reduced model, four electrons are transferred to dioxygen through sequential one-electron oxidations of heme as s iron ion, the Cub ion, the heme a iron ion, and one of the bimetallic center s Cua ions. The sequence of electron transferal differs in the mixed valence model, and a tyrosine radical (tyr) is generated. The proposed formation of a tyrosine radical during catalytic turnover arises from the known post-translational modification in most CcO s in which a covalent bond is formed between the his240 ligand of Cub... 

A condition where metal ions within a coordination complex or cluster are present in more than one oxidation state. In such systems, there is often complete delocalization of the valence electrons over the entire complex or cluster, and this is thought to facilitate electron-transfer reactions. Mixed valency has been observed in iron-sulfur proteins. Other terms for this behavior include mixed oxidation state and nonintegral oxidation state. 

The mixed-valence iron oxides provide an experimental test-bed for studying the evolution of charge-transfer processes from the localized-electron to the itinerant-electron regimes. Moreover, it is possible to monitor the influence of the charge transfer on the interatomic magnetic coupling since the iron ions in oxides carry localized magnetic moments. 

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... 

---