Mixed-valence compound

The discussion in the preceding sections has been concerned with compounds which show a single valence or oxidation state. In this section some compounds which have iron in more than one oxidation state, usually iron(II) and iron(III), will be considered. Mossbauer spectroscopy can be very useful in determining the structural and electronic properties of such mixed valence compounds. 

Robin Day (1967) have classified mixed valence compounds on the basis of the eneigy required to transfer an electron between the low valence site and the high valence site. In Class I compounds, the ground state M(II)/ M (III) electronic configuration (where M and M refer to different iron(II) and iron(lll) sites in a compound) is much lower in eneigy than the excited state M(III)/M (II) configuration which may result from an intervalence electron transfer. In this type of compound the two iron sites may be 

As mentioned above, the results obtained in the study of a mixed valence compound depend upon the characteristic time associated with the measurement. For Mossbauer spectroscopy an upper limit on this characteristic time is given by the mean lifetime of the nuclear excited state involved in the Mossbauer transition, which in the case of Fe is 97.8 nanoseconds (see Chapter 5 for a more complete discussion of the timescales associated with a Mossbauer spectroscopy measurement). The Mossbauer spectral parameters will depend upon the typical electronic environment that the nucleus senses during this characteristic measurement time. 

K2Pt(CN)4Bro.3o-3H20 Class III Molecular metal  

Electronic properties associated with/-electrons receiving much attention recently are 

Robin Day (1967) have proposed a classification of mixed-valence compounds based on the valence delocalization coefficient, a, the magnitude of which depends on the energy difference between the two states and Mb, where A 

Mixed valency occurs in minerals (e.g. 63 ), metal-chain compounds, dimers and oligomers and metal complexes, and even in organic and biological systems (Brown, 1980 Day, 1981). Among the dimeric and oligomeric metal complexes exhibiting mixed valency, the pyrazine-bridged Ru (II, III) ammine complex. 

FbjO has the inverse spinel structure, with all the Fe ions and half of the Fe ions located in octahedral sites (B sites) in the oxygen network and the remaining half of the Fe ions located on tetrahedral sites (A sites). It undergoes a ferrimagnetic-paramagnetic transition around 850 K and another transition around Ty = 123 K (Verwey transition). The material is a semiconductor both above and below the Verwey 

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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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