Valency mixed-valent

High-valent iron also occurs in -nitrido bridged dimers with linear [Fe °-N=Fe" ]" and [Fe =N=Fe ] " cores [209, 210] (and references therein). Such compounds have been prepared first by thermolysis [247] or photolysis [248] of iron(III)-porphyrin complexes with an azide ligand, (N3). Mixed-valent iron-nitrido porphyrin dimers exhibit valence delocalization as can be inferred from the... 

The electrostatically favored cation (Li) and anion (RE) arrangement implies the presence of two different E-, Si- and Li sorts, which has been established by solution and solid-state NMR spectroscopy. The electronic structures of the mixed-valent pnictides 10 and 11 have been simply described as electron-deficient clusters with delocalized framework electrons. Formally the latter consist of two low-valent anediyl moieties RE and eight andiides (RE)2- (E = P, As). The relatively large E-E distances of >4 A exclude the occurrence of localized E-E bonds. However, delocalization of the cluster valence electrons is achieved without Li-Li bonds via Li-mediated multiple bonding. Evidence for this has been seen in the NMR spectra (31P, 7Li, 29Si), which are in accordance with the electron delocalization model (see later discussion). 

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. 

There are several interesting families of inorganic mixed-valence compounds that we have not discussed here (see Yvon, 1979 McCarley, 1982). For example, there are metal-cluster compounds such as the Chevrel phases, M,jMo6X8(X = S or Se) and condensed metal-cluster chain compounds such as TlMojScj, TijTe, NaMo O and M PtjO. TTF halides and TTF-TCNQ complexes (Section 1.9) constitute molecular mixed-valent systems in which the mixed valency is associated with an entire molecule the charge on TTF in such compounds is nonintegral. The structure of TTF-Br(, 79 and... 

In the following we present a sample of calculation intended as a preliminary illustration about the use of the discussed method in the problems of mixed valence. The chemical literature of mixed valent transition metal complexes is huge and the connection with the vibronic formalism is widely acknowledged [17]. However,... 

Mixed valence" was defined by chemists as follows If ions (or molecules) of two different formal charges or valences occupy the same crystallographic site, then an intermediate "mixed-valent" state (an average of the two valences) is assigned to the ions of molecules at that site [38]. 

The complex SmCp adds on CsHs forming a trivalent and mixed valent product CpjSmCp, and Cp SmCpSmCp, respectively. In the mixed valent product there is a deficiency of C5H6. In the complex Cp Sm"1(/4-Cp)Sm"Cp2 the two valence states have been assigned on the basis of spectroscopy [141] which has been confirmed by X-ray analysis. The Sm(III) coordination geometry is trigonal. The bridging Cp is symmetrically bound to Sm(III) and two atoms are closer to Sm(II) at distances of 2.986(8) and 3.180(9) A, respectively. 

A survey of the literature from 1980 to 1993 shows that over 2500 articles have been published during this period that contain mixed valence or mixed valent in the title. From 1994-2003, Mixed Valence was in 3 9 8 8 titles and there were an additional 1095 that contain Mixed Valent . Four of these are reviews. Thus, the 1967 statement of Robin and Day that stated mixed valence chemistry represented a neglected class ofinorganic compounds is no longervalid. Mixed valency, the... 

Mixed valency is not limited to consideration of individual ions in different oxidation states. Delocahzed systems also exist that exhibit properties of mixed valency, as in many organometalhc mixed valence compounds. For example, phthalocyanine, Pc (1), sandwich compounds of the lanthanides, Ln(Pc)2, are best described as containing the two Pc anionic ligands in mixed valent charge states of —2 and — 1 with the metal ion Ln +. 

In these compounds, as in the mixed valence copper sulfides,the nonmetallic moiety is the mixed valent species. Superconductivity also is known with the sulfides in the Chevrel Phases. In 1991, the pure fullerene phase of carbon,... 

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