Mixed valence complexes research

There have been only a handful of studies on exchange coupled ruthenium dimers and yet the information available on metal-metal coupling is potentially as valuable as that obtained from mixed-valence complexes. The reason for this lack of activity has been the greater familiarity of researchers with the chemistry of Ru(II) bound to 7r-acid ligands. The synthetic pathways to complexes of this type are well explored (7) and possess the tremendous advantages of stability and ease of handling. Ru(II) complexes that incorporate anionic n donor... 

This review of mixed valence copper(I)/(II) systems has clearly established the predominance of the class I Robin and Day behaviour (Table 17), 360-362 but equally has shown how few copper class II or III systems have been well defined. This particularly applies to the class II systems, which can still be considered well-defined coordination complexes, with the electronic properties of these systems in the solid state and in solution. This suggests a fruitful area of research in these copper(I)/(II) mixed valence systems, especially of class II behaviour. 

Returning now to the comproportionation equilibrium, it is interesting to notice that for large systems (2 mm co) the comproportionation constant tends towards a statistical limit of 4 (see below). Then a single electrochemical wave is observed, a fact which has frequently dissuaded researchers from further studies on such binu-clear complexes. However, even in this apparently unfavorable case, the proportion of mixed-valence species at half-oxidation (reduction) is, according to Eq. (3), a comfortable 50 %, nevertheless allowing a meaningful correction. 

In the specific case of scandium, mono- and mixed-valence species can also be isolated together with divalent complexes for instance, a Sc organometallic compoimd could be obtained imder relatively mild conditions. Finally, the author describes the few knovm zerovalent bis(arene) rare-earth complexes which have been obtained by co-condensation of arenes or heteroarenes with metal vapors. In his conclusion, F. Nief notes that the low-valence molecular chemistry of rare earths, which was once thought to be restricted to divalent samarium, europium, and ytterbium, has been extended to several other rare earths, as well as to lower valence oxidation states. It is the opinion of the author that this research area is likely to find fascinating developments in a near future. 

This is certainly one of the most important design considerations. Partial filling of the Brillouin zone is necessary for metallic conduction. In fact, even in the early TCNQ research at duPont in the 1960 s, higher conductivities were achieved from complex TCNQ salts than with simple salts [88-90] (Table 3). The last two entries in Table 3 are really semiconductors with an activation energy for conduction, but the room-temperature resistivities are quite low. It is clear, however, that partial CT is essential it is also clear that mixed valence (in the sense of the Robin-Day [91] class IIIB) must also be achieved, i.e. one cannot have discrete valences at discrete sites (as, e.g. in CS2TCNQ3, which is a complex salt, but has low conductivity because it has a ISCA3D lattice). 

In addition to the above normal valence materials, recent research has uncovered a number of mixed valence and apparent subvalent halides. For example, discreet compounds with halogen to rare earth ratios of2 200, 2 167, and 2 140 have been observed for chlorides and bromides of Dy, Yb, Ho, and Sm. These are really mixed valence 3-E, 2-E compounds with formulas such as DysCln, Yb6Cli3, and SmnBr24. The complex structures of these materials have been the subject of considerable research. 

In view of the rapid development in the synthesis of metal alkynyl complexes and the inherent advantage of the rigidity of such systems, attempts have been made by researchers to synthesize mixed-valence metal alkynyl [210, 211] and mixed-metal alkynyl complexes [64]. The incorporation of different metal centers to the organic spacer, which is usually achieved by the metalloligand approach, may provide a good handle to investigate the electron transfer and communication across the molecule. Isolation of mixed-valence homometallic species have... 

Physical properties of binary or ternary Ru/Ir based mixed oxides with valve metal additions is still a field which deserves further research. The complexity of this matter has been demonstrated by Triggs [49] on (Ru,Ti)Ox who has shown, using XPS and other techniques (UPS, Mossbauer, Absorption, Conductivity), that Ru in TiOz (Ti rich phase) adopts different valence states depending on the environment. Possible donors or acceptors are compensated by Ru in the respective valence state. Trivalent donors are compensated by Ru5+, pentavalent acceptors will be compensated by Ru3+ or even Ru2+. In pure TiOz ruthenium adopts the tetravalent state. The surface composition of the titanium rich phase (2% Ru) was found to be identical to the nominal composition. 

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