Chevrel compound, structure

The electronic structure of Chevrel compounds is similar to that in Fig. 7.3(a), but with different numbers of states in the upper and lower bands. Mo-Mo bonds split the d states into two bands (Nohl, Klose and Andersen, 1982). The lower band contains 24 states per MogSeg cluster, because there are 12 Mo-Mo bonds in each cluster (the 12 edges of the Mog octahedron). Each Mo contributes six electrons to the bands, for a... 

Fig. 5. Structure of Chevrel compounds, A,Mo6Ch8 (Ch = S, Se, Te) (a) Mo, Chg building block, (b) structure of Chevrel compounds where the A cation is shown by the closed circle.

Figure 4. Structure of one of the Chevrel compounds PbMo6Ss. This drawing was kindly supplied by M. J. Sienko of Cornell University. ((%) Mo (O) S ( ) Pb)...

Figure 16-21 The idealized structure of a representative Chevrel compound.

The different classes of Ru-based catalysts, including crystalline Chevrel-phase chalcogenides, nanostructured Ru, and Ru-Se clusters, and also Ru-N chelate compounds (RuNj), have been reviewed recently by Lee and Popov [29] in terms of the activity and selectivity toward the four-electron oxygen reduction to water. The conclusion was drawn that selenium is a critical element controlling the catalytic properties of Ru clusters as it directly modifies the electronic structure of the catalytic reaction center and increases the resistance to electrochemical oxidation of interfacial Ru atoms in acidic environments. 

Cu4Mo6Seg. The triclinic structure of these compounds has been described by McGuire et al. (2006). It contains nearly regular Mo6 clusters the Cu atoms fully occupy four sites between faces of two adjacent Mo6Se8 clusters. This compound is a member of a series of MexMogX8 Chevrel phases. Preparation and properties have been discussed. A Cu intercalation apparatus was built for... 

The Chevrel phases are structure containing clusters as building blocks and they may be considered as phases having characteristics at the borderline between those of compounds containing molecular-like units and of solids containing infini-tively extended units. Such phases have been described in 4.4.4.7 together with Mo6X8 clusters and the products of their condensation. 

X-ray crystallography, 40 20-21 synthetic models, 40 23-48 xanthane oxidase, 40 21-23 chalcogenide halides, 23 370-377, 413 Chevrel phases, 23 376-377 metal-metal bonding, 23 330, 373 structural data, 23 373-376 as superconductors, 23 376 synthesis, 23 371-372 chloride, 46 4-24, 35-44 heterocations of, 9 290, 291 cluster compounds, 44 45-46 octahedral, 44 47-49, 53-63 electronic structure, 44 55-63 molecular structure, 44 53-54 synthesis, 44 47-49 rhomboidal, 44 75-82 solid-state clusters and, 44 66-72, 74-75, 80-82, 85-87 tetrahedral, 44 72-75 triangular, 44 82-87 cofactor, 40 2, 4-12 anaerobic isolation, 40 5 molybdopterin and, 40 4-8 reduced form, 40 12 synthesis, 40 8-12 xanthine oxidase, 45 60-63 complexes... 

In passing, it should be noted that Michel and Raveau had carried out electrical measurements on several alkaline-earth substituted La2Cu04 compounds and presented (140) their experimental results in a comprehensive review during 1984. Their work, however, was primarily structural with a focus on electrical and magnetic properties above 50 K for this reason they missed observing the superconducting transition at 30 K. It is this author s belief that these Cu-O phases should be called the "Raveau-Michel" phases in much the same way as the molybdenum sulfide superconductors are called the "Chevrel" phases. 

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... 

If steric effects in solid-state cluster compounds could be eliminated by the isolation of discrete molecules with the same cluster units as Chevrel phases, a part of the preceding problem would be solved. The structures of the cluster molecules in the isolated state free from the crystal packing effects are most suitable for such discussion. The geometry of free molecules in vacuum cannot be known from X-ray structure determination, and usually it is difficult to measure crystal packing effects in molecular crystals. In the case of metal complexes with sym-... 

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