Intermediate valence state

Rare-earth ions inserted in the tetraborides have the 34- oxidation state, except for CeB4 and YbB4 (see Fig. 2). The abnormal volume contraction for the CeB4 unit cell can be explained by the presence of some Ce ions . Recoilless y-ray emission spectra and magnetic measurements indicate that ytterbium in YbB4 has an intermediate valence state as in YbAl3... 

The rare earths in their dodecaborides have the 3 + oxidation state except for Yb and Tm which have an intermediate valence state. A recoilless y-ray emission spectrum study of TmB,2 shows no magnetic ordering at 1.35 K the spectra of YbB,2 reveal no magnetic structure to 1.35 K. The compounds HoB,2, ErB,2 order antiferromagnetically, and ZrB,2 and LuB,2 become superconducting < 5.8 K and < 0.48 K, respectively. ... 

The oxidation of tartaric and glycollic acid by chromic acid also induces the oxidation of manganous ions. In the presence of higher concentrations of manganese(II) the rate of oxidation of the acids is diminished to about one-third of that in the absence of manganous ions. The decrease of the rate has been attributed to manganese(II) catalysis of the disproportionation of the intermediate valence states of chromium probably chromium(IV). 

He suggested that the ionic formulas, like the nonionic formulas, "represent formulations of extremes" and that no bond across the ring is required. Using the hypothesis of the motions of valence electrons, as developed by Stark and Kossel, Arndt suggested the possibility of intermediate valence states (Zwitterstufen) as well.32 Independently, Robinson proposed possible electronic shifts in pyrones and similar systems, but he did not state the idea of a definite "intermediate state" of the molecule between the ionic and uncharged formulas.33... 

All these data verify that in real systems, the rate of electron transfer between components of a conductive chain is high. There are states of a mixed valence. Enhanced electric conductivity and other unusual physical properties are widespread among those inorganic or coordination compounds that contain metals in intermediate -valence states. In cases of organic metals, nonstoi-chiometric donor/acceptor ratios provide even better results. For example, the salt of (TTF)i (Br)oj composition displays an electric conductivity of 2 X 10 cm while (TTF)i(Br)i salt does not... 

The complexity of the situation may be illustrated by tracing the development of our knowledge of chromia catalysts. It is now clear that Cr ions on the surface may occur in valence states from Cr11 to CrVI all intermediate valence states have been shown to occur, and all are possible in a regime of temperature and gas composition where the thermodynamically stable bulk phase is Cr2C>3. 

The position of the ASV peak on the voltage scan reflects the nature of the ion being reduced, and for complex ions the peak position moves to more negative potentials as stability increases. In some cases formation of intermediate valency states (e.g. in chloride solution, Cu2+ — Cu+ — Cu°) results in split peaks. Adsorption of species (e.g. colloidal particles, surfactants) on the mercury electrode also causes peak movement (generally in an anodic direction). 

Opacity of mixed-valence minerals. The opacities of many end-member Fe2+-Fe3+ oxide and silicate minerals result from electron hopping between neighbouring cations when they are located in infinite chains or bands of edge-shared octahedra in the crystal structures. Opaque minerals such as magnetite, ilvaite, deerite, cronstedtite, riebeckite and laihunite owe their relatively high electrical conductivities to thermally activated electron delocalization, contributing to intermediate valence states of iron cations which may be detected by Mossbauer spectroscopy. 

Intermediate Valence States in Oxidation-Reduction Reactions... 

The analogue of free radicals for inorganic ions would be intermediate valence states, and many of the kinetic results on redox reactions have required the postulation of unstable valence states for inorganic ions. One of the classic studies of this type is the slow reaction 2Fe + Sn++ 2Fe " + Sn +. The reaction is very slow in HCIO4 solution but is strongly catalyzed by C1-. At high Fe +/Sn++ ratios, the rate law has the form... 

Intermediate valence states are frequently of importance as catalysts in redox systems. Thus Fe + will oxidize 1 very slowly in dilute acid solutions to form Fe++ and Similarly, the reaction between Cr207 (or IlCrOr) and I in acid solutions is extremely slow. " If, however, Fe++, I , and Cr20f are mixed together, a very rapid oxidation of I to la " occurs, accompanied by the oxidation of Fe" + to Fe +. In this system Cr must certainly pass through either a 4+ or a 5+ valence state, and current interpretations favor the latter. Further evidence for a Cr(V) state comes from the studies of the exchange between Cr(VI) and Cr(III) in 0.16 M HCIO4 solutions. It is found that the rate law is... 

For such reactions an induction factor IF is defined as the number of equivalents of reductant oxidized per equivalent of inducer (here Fe ) oxidized. In essence reducing agent and inducer compete with each other in the further reduction of the intermediate valence states of oxidizing agent. 

A number of intermediate valence states of chromium are involved in this reaction as orange Cr ultimately is reduced to green Cr ". The course of the oxidation can be followed by these color changes. 

The fate of Cr(VI) within the cell is not determined by thermodynamics alone. There are several cases, e.g., isocitrate (discussed below), of reducing agents which have redox potentials adequate to accomplish the reductions of Cr(VI) but which do so at an insignificant rate. Thus of key concern will be a consideration of the kinetic factors involved in the three electron conversion of Cr(VI) to Cr(III). To appreciate these factors we need to consider the structures of both Cr(VI) and Cr(III) and the structure, stability and lability of the intermediate valency states Cr(V) and Cr(IV). 

A Liii XANES spectrum in the ytterbium region of YbCuGa exhibits two distinct peaks separated by 7eV (Adroja et al. 1990), indicating an intermediate-valence state of the ytterbium atoms. This is in agreement with the magnetic behavior. 

In contrast to the pnictides, the equilibrium volumes of the Yb chalcogenides are accurately described assuming the divalent/ configuration for Yb. As pressme is applied to the Yb chalcogenides, the configuration becomes more and more favourable, and eventually a transition to an intermediate valence state occurs. Valence transitions in lanthanide systems will be discussed in the next section. 

A charged state is obtained if the upper row is occupied by two electrons each while the alternant sites in the lower row are left unoccupied. The total number of electrons with x = 0 is 2L, but there are (2L) sites in the system. Apparently, the occupancy of the x = 0 states is just an indicator of a possible instability. We know from the BCS model that the phonon interactions contribute to pair formation. This may lead to an energetically favored charged state if the intermediate valence state is missing. ... 

Alternatively, we may occupy the upper orbitals in Figures 17.9 and 17.10 by one electron with spin up, while the lower orbitals are occupied by one electron with spin down. Adding the other possible wave function of this type with opposite spins leads to a spin-coupled state, since the spins are different on adjacent sites. Thus, if the intermediate valence state is present at a low energy, spin coupling occurs, as in the undoped cuprates. It should be noted that we are dealing with chemical effects, which are not easily calculated and not easily understood. What is important is that in addition to these two possibilities, an electron pair current state is possible if the charged state and spin-coupled state are almost degenerate. 

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