Mixed Valency Compounds

Molecules are known which contain P atoms formally in different valency states, for example, (3.30a) through (3.30c). What are styled as onium-ate compounds may sometimes be included in this category (3.30d). Equilibria involving the same P atom in alternative valency states are more common, however (see tantomerism, etc. below). 

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Tominaga K, Kliner D A V, Johnson A E, Levinger N E and Barbara P F 1993 Femtosecond experiments and absolute rate calculations on intervalence electron transfer of mixed-valence compounds J. Chem. Rhys. 98 1228—43... 

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). 

These mixed-valence compounds have magnetic moments around 4/tB, indicating an S = 3/2 (quartet) ground state, in keeping with their ESR spectra, which resemble those of Cr3+ compounds with a big zero-field splitting (gj = 4, g = 2) [99]. 

All the trihalides are mixed valence compounds. PtF3 is isostructural with PdF3- PtX3 (X = Cl, Br, I) cannot be made by straightforward thermal decomposition of PtX4 [22] under open conditions but by routes involving continuous decomposition and formation under closed, equilibrium conditions. 

Ptl4— has been identified in the unusual (MeNFt3)4Pt3In, a mixed-valence compound with Ptl4, Ptl and Pt2Is ions all present (K2PtI4 has not definitely been confirmed) [27],... 

A dynamic model for mixed valence compounds. K. Y. Wong and P. N. Schatz, Prog. Inorg. Chem., 1981, 28, 369-449 (98). 

Theory and experiments on valence delocalisation in mixed-valence compounds. P. Day, Comments Inorg. Chem., 1981,1,155-167 (23). 

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... 

Consider now NajW03 or LiTi204. One might expect to find W(V)-W(VI) and Ti(III)-Ti(lV) MMCT. However, in these compounds all metal ions are equivalent and the d electrons available are spread out in a conduction band. The bronzes NajW03 are metallic. They become superconducting at 6K, whereas LiTi204 becomes superconducting at even 13 K [59]. Here we meet the central problem of mixed-valence compounds [60] which we will postpone till Sect. 5. 

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. 

Mixed-valence compounds are divided into three classes following proposals by Robin and Day [98] and Hush [99]. The basis for this classification is the... 

Whereas the relation between MMCT and mixed-valence compounds is immediately clear, this is not the case for MMCT and semiconductors. Their relation will be discussed in the next paragraph. 

See e.g. Brown DB (ed) (1980) Mixed-Valence Compounds, Reidel, Dordrecht... 

Platinum is unusual in the extraordinary variety of mixed-valence compounds that it forms. Many of these have novel spectroscopic, electronic, and structural properties. 

The localized-valence halogen-bridged platinum(II)/platinum(IV) complexes (175) are of interest because of their highly anisotropic electrical and optical properties. The complexes are characterized by broad and intense intervalence bands in their electronic spectra.542 Partial oxidation of [PtCl(dien)]Cl with iodine yields the mixed valence species [PtI(dien)][PtI3(dien)]I2,543 but when 4Me-dien is used, the mixed valence compound is only obtained if it is trapped quickly, before isomerization of the meridional platinum(IV) complex to a facial form.544 Although these species nominally have localized valences, the extent of delocalization increases as the bridging halide... 

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