Intervalence transition

Mixed valency of this sort is the cause of the reflective, gold colour of Nao.3W03. In this system, like the MnfTc ion described above, electrons are excited optically following photon absorption from a ground-state electronic configuration to a vacant electronic state on an adjacent ion or atom. The colour is caused by a photo-effected intervalence transition between adjacent WVI and Wv valence sites ... 

Such intervalence transitions are characterized by broad, intense and relatively featureless absorption bands in the UV, visible or near infrared (NIR). 

In the spectra of [V203(pmida)2] and [V203(nta)2]3-, a new band around 1000 nm was assigned to the intervalence transition.814 The spectrum of [V9Ol6(btda)4]7 is the same in solution and in the solid.813 The band at 900 and a shoulder at 1020 nm corresponded to two components of the intervalence band. 

The intervalence transitions of doped [M(oep)(L-L)] m+ polymers (M = Fe, Ru, or Os and L-L = pz, dabco, or 4,4 -bpy) have been studied. The metal-metal coupling and the intensity of the intervalence transition increase in the order dabco < 4,4 -bpy < pz, and the energy of the intervalence band decreases in the same order. Within a series with the same bridging ligand, the strength of metal-metal coupling and the intensity of the intervalence transition increase in the order Fe < Ru < Os, and the energies of intervalence bands decrease in the same order. With [Os(oep)pz] m+, an intense and relatively sharp electronic transition is observed at 2000 cm 1 in the IR (563-565), which is reminiscent of a similar transition in [(NH3)5Os(pz)Os(NH3)5]5+ (Section IV,B,l,b). 

The Pt + Pt intervalence transitions of such chain complexes occur in the regions 25,000-18,200 cm 1, 23,600-14,300 cm 1 and 20,600-7,500 cm 1 for chloro-, bromo-, and iodo-bridged complexes, respectively, the trend Cl > Br > I being the reverse of that of the conductivity of the complexes. The transition wavenumbers may be determined either by Kramers-Kronig analysis of specular reflectance measurements or from plots of the excitation profiles of Raman bands enhanced at or near resonance with the Pt I-PtIV intervalence band. The maxima have been found to be related to the Pt —PtIV chain distance, the smaller the latter the less being the intervalence transition energy (3). 

Homonuclear intervalence transitions involve single elements in different oxidation states. For example, electron transfer between Fe2+ and Fe3+ ions located in adjacent octahedral sites A and B, may be represented by ... 

Heteronuclear intervalence transitions involve different cations. An example involving the octahedral Fe2+ and Ti4 cation pair, may be depicted by... 

Optical absorption spectroscopy No intervalence transitions in visible region One mixed-valence transition in visible region absorption bands intensify at elevated pressures and low temperatures One or more mixed-valence transitions in visible region temperature lowers intensity of absorption bands, pressure intensifies Opaque metallic reflectivity in visible region... 

Bums, R. G. (1981) Intervalence transitions in mixed-valence minerals of iron and titanium. Ann. Rev. Earth Planet. Sci., 9, 345-83. 

Figure 1. Potential energy as a function of reaction coordinate for a self-exchange reaction. AE, energy barrier for thermal electron transfer (weak coupling) AE2, energy of an intervalence transition which is possible for the system.

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 weak coupling cases, Hush showed that the intensity of an intervalence transition was related to the extent of coupling between donor and acceptor. The derivation (11) begins by considering the theoretical expression for oscillator strength,... 

Because the ground state is delocalized between metal ions, it is not strictly appropriate to describe the intervalence transition band of a class III complex as a metal-to-metal charge transfer transition. Nevertheless, the MMCT designation will be used for the sake of simplicity. 

The dependence of intervalence transition energy on the activation barrier to thermal electron transfer in Eqs. (1) and (3) allows the solvent dependence of intervalence transitions to be understood in terms of relationships developed for the Marcus Theory of electron transfer (5, 6). 

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