Transitions mixed-valence

For mixed-valence bimetallic systems, Robin and Day distinguished three classes depending on the amount of metal-to-metal interaction [17]. Figure 6 is a plot of nuclear configuration vs. energy for the three classes of mixed-valence compounds. If there is essentially no interaction between the metal centers, it is called a class I system. Compounds of this type have properties that are a simple combination of Ihe properties of the two independent metal centers. Class I compounds do not exhibit IT transitions. Mixed-valence compounds that have some limited degree of interaction between the metal centers are considered to be class n. Class n systems still have a localized valency and can be described as... 

The reduction of molybdate salts in acidic solutions leads to the formation of the molybdenum blues (9). Reductants include dithionite, staimous ion, hydrazine, and ascorbate. The molybdenum blues are mixed-valence compounds where the blue color presumably arises from the intervalence Mo(V) — Mo(VI) electronic transition. These can be viewed as intermediate members of the class of mixed oxy hydroxides the end members of which are Mo(VI)02 and Mo(V)0(OH)2 [27845-91-6]. MoO and Mo(VI) solutions have been used as effective detectors of reductants because formation of the blue color can be monitored spectrophotometrically. The nonprotonic oxides of average oxidation state between V and VI are the molybdenum bronzes, known for their metallic luster and used in the formulation of bronze paints (see Paint). 

Oxide materials in the colored state are usually mixed-valence compounds with a variable range of composition. The color usually arises from low-energy intraband electronic transitions. 

The organization of this paper is as follows. In Sect. 2 we will discuss at length MMCT for those cases in which the metal ion which is reduced in the MMCT transition (the electron acceptor) is an ion with configuration (e.g. Ti t+, y5+ Nb ", In Sect. 3 we will mention several other cases without aiming at completeness. In Sect. 4 we will review the work by McGlynn et al. [5], since it bears a clear relation to the phenomena described in the foregoing sections. In Sect. 5 we will shortly enter into the problem of the mixed valence compounds which is in this aspect undoubtedly in order. In Sect. 6 the importance of MMCT transitions for semiconductors will be mentioned. Finally Sect. 7 will present the consequences of MMCT excited states for radiative and... 

It is interesting in this aspect to note that the 5s 5s5p transitions of Sb(III) in SbClg " are very similar in the solid model compounds Cs2NaLnCl6 Sb(III) (Ln = La, Y, Sc) [72], in the mixed valence compounds of the type CsjSbClg, and in aqueous solutions [73]. 

After a consideration of optical transitions in which MMCT plays a role, and after a characterization of the excited states involved, a short review of mixed-valence compounds and their spectroscopy is in order. For more extended reviews we refer to Refs. [60,97], At least 40 elements of the periodic table form mixed-valence species which are of importance in solid state physics and chemistry, inorganic chemistry, materials science, geology and bioinorganic chemistry. It is usually their colors which are their most striking property (see also above), but they have more intriguing properties, for example electrical and magnetic properties. 

An interesting aspect of Au oxidation states is provided by the investigation of the pressure-induced transition from the mixed-valence state of Au(l)/Au(lll) to the single valence state of Au(ll) as described for M2[Au(l)X2][Au(lll)X4] (M = Rb, Cs X = Cl, Br, 1) [385, 386]. The valence states of Au(l) and Au(lll) at ambient pressure were clearly distinguishable. With increasing pressure, the doublets gradually increase their overlap. Finally, the Au Mbssbauer spectrum of CS2AU2I6 shows, at 12.5 GPa of applied pressure, only one doublet which was associated with Au(II). 

The ferrocenyldiphynylpropargyl cation, 77, has an intrinsic delocalization nature exhibiting a valence tautomerization band at 856 nm, and its nucleophilic trapping reactions give rise to the formation of ferrocenyldiphyenylallenes (173). The bis(acetylide) mixed-valence complexes of ferrocene and the Ru complex moiety, 78, also behave as a fulvene-cumulene structure, 79, showing a u(M=C = C—C) band at 1985 cm-1 (174). Related alleylidene and cumulenylidene complexes of transition metals have been reviewed by Bruce (175). 

Examples are tending to be more sophisticated and complex in form. For example, a dinuclear complex featuring a bridging phosphinate and phenolate in addition to peroxide (221) has been reported,966 as a model for phosphodiester systems. Apart from dicobalt(III) systems, a mixed-valence CoII,ni di-/i-superoxo complex (222) has been prepared.967 Transition between the three redox states CoII,n, Co11,111, and Co111111 is electrochemically reversible. 

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 ... 

X-Ray photoelectric ionization is believed to take place in a time interval of about 10-18 s. Therefore separate XPS peaks are possible for atoms if the lifetime of the asymmetric electronic state is greater than about 10 18 s, whether or not the atoms are structurally equivalent. We may represent the ground state of a localized mixed valence compound (involving two metal atoms differing in oxidation state by one unit) by the following formula, where the dot represents the extra valence electron M—M. The two possible XPS transitions can then be represented as follows, where the asterisk indicates core ionization,... 

The first transition would be expected to be of higher energy than the second from simple atomic charge considerations. Because the two atoms are of equal abundance, the two peaks have essentially equal intensities. Unfortunately, the observation of two XPS peaks does not rule out the possibility of delocalized valence electrons in the ground state. Two transitions are expected even in that case because of polarization of the excited state by the core ionization 123 The ground state of a delocalized mixed valence compound can be crudely represented by the formula M-M, where the intermediate position of the dot indicates that the odd valence electron is equally shared by the two metal atoms. The two XPS transitions can then be represented as follows,... 

Notice that the excited states are similar to those formed from the localized ground states. Again the first transition is of higher energy than the second. Hush has made a theoretical study of these transitions123. He concluded that, in the case of symmetrical delocalized mixed valence complexes, the two XPS peaks will occur at energies... 

With regard to the latter point, the absence of a mixed valence transition in the oxidized low coverage polymer case is an important point. No mixed valence transtiion was observed over the whole range of oxidation (0-100%) studied. This indicates that whereas the D22+ aggregate is stable under these conditions, the mixed valent dimer analog, D2+, is not. At least in these polymeric matrices, therefore, the stoichiometric requirement for observation of the mixed valence state appears to involve (D2+)n where n > 2. 

Optical charge transfer (CT) is commonly observed in un-symmetrical molecules or molecular complexes in which there are sites of distinctly different ionization energies and electron affinities. The origin and properties of optical charge transfer transitions provide the basis for this account. A convenient place to begin chemically is with mixed-valence compounds and two examples are shown below (1-3). In the first (eq 1), the sites of different oxidation states are held in close... 

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