Mixed-valence metals

These results suggest that the critical factor in the substrate-mediated intermolecular interactions which occur within the close-packed DHT layer is the inherent strong reactivity of the diphenolic moiety with the Pt surface. The interaction of adsorbates with each other through the mediation of the substrate is of fundamental importance in surface science. The theoretical treatment, however, involves complicated many-body potentials which are presently not well-understood (2.). It is instructive to view the present case of Pt-substrate-mediated DHT-DHT interactions in terms of mixed-valence metal complexes (2A) For example, in the binuclear mixed-valence complex, (NH3)5RU(11)-bpy-Ru(111) (NH 3)5 (where bpy is 4,4 -bipyridine), the two metal centers are still able to interact with each other via the delocalized electrons within the bpy ligand. The interaction between the Ru(II) and Ru(III) ions in this mixed-valence complex is therefore ligand-mediated. The Ru(II)-Ru(III) coupling can be written schematically as ... 

Even the basic properties of mixed metal or heteronuclear complexes and mixed valence metal complexes have not been studied, so that their catalytic effects are far from dearly understood. 

Other mixed-valence metal complexes have been published that display electrochromic behavior, among them bipyridyl ruthenium complexes linked through dioxolene containing bridging ligands, such as 3,3, 4,4 -tetrahydroxybi-phenyl, 3,3",4,4"-tetrahydroxy-p-terphenyl, and 9-phenyl-2,3,7-trihydoxy-6-flu-orone [115,116] (Chart 3). Many of the redox processes that lead to frequently... 

Mixed-valence metal string complexes of gold... 

Structures of two mixed-valence metal string molecules (large filled circle, Au large open circle, R group small filled circle, C small open circle, P bond... 

Selective perturbation of metal-metal coupling by either inner-sphere or outer-sphere manipulations will continue. Aside from increasing our understanding of metal-metal superexchange coupling, these studies will probe the nature of solvent-solute interactions and perhaps be applicable to the design of molecular devices. The synthesis of novel nonlinear optical materials that incorporate mixed-valence metal systems will be the subject of much interest 100). 

It is clear that much work remains to be done to extend our understanding to polax surfaces of transition metal oxides in which the cations have partially filled d orbitals. An especially challenging issue is related to mixed valence metal oxides, such as Fe304, in which the cations exist under two oxidation states. In addition, considering the rapid development of ultra-thin film synthesis and characterization, a simultaneous effort should be performed on the theoretical side to settle the conditions of stability of polar films. More generally, on the experimental side, it seems that one of the present bottlenecks is in a quantitative determination of the surface stoichiometry, an information of prominent interest to interpret the presence or absence of reconstruction. 

The encounter complexes exhibit high degrees of charge-transfer [20, 91], and on the basis of absorption and emission data electronic coupling matrix elements for similar complexes (exciplexes) have been determined [205] which are comparable to those of mixed-valence metal complexes commonly used as prototypical models for the bridged-activated complex in inner-sphere electron transfers [2, 26, 197]. Accordingly, we ascribe the unusually high rate constants, their temperature-independence, and their on-Marcus behavior to an inner-sphere electron transfer process [31]. 

The term charge resonance is adopted from studies with mixed-valence metal complexes and cation-radical-neutral 7t-dimers (D+, D <- D, D+) [150]. 

On the other hand, the appearance of a near-infrared absorption for an electrochemically or otherwise generated intermediate can be viewed as indicating a potentially mixed-valence situation. For instance, the stepwise chloride-dissociative reduction of compounds [Cl(C R )M( i-L)M(C R )Cl], w = 5 for M = Rh or Ir, n = 6 for M = Ru, may lead, after addition of three electrons (and the loss of two CP ions) to a species [(C R )M()i-L)M(C R )] with a distinct near-infrared absorption. In conjunction with the EPR spectra showing large g factor anisotropy (see Section 3.4) it can be concluded that these are mixed-valence metal intermediates and not radical complexes, obtained... 

Laidlaw, W.M., Denning, R.G., Verbiest, T., Chauchard, E., Persoons, A. Large second-order optical polarizabilities in mixed-valency metal complexes. Nature 363, 58-60 (1993)... 

The electrochemistry of POMs is characterized by reversible multistep redox processes resulting in the formation of mixed valence species and electron self-exchange between mixed-valence metal sites (e.g.. Mo ) via electron hopping. Extensive reduction of POMs leads to the formation of heteropolybrown species in which such distinct ionic sites (e.g., W and W" ) can coexist. 

A prerequisite for such estimates is that the encounter complex is sufficiently long lived for hv to he measured. Another condition is that the resonance energy H at the point of intersection of the two surfaces is small (Figure 5.10). Both conditions are met in certain oligonuclear mixed-valence metal complexes. 

The dominant line at es 377 MHz is typical of mixed valence metallic-like manganites and corresponds to a manganese ion state with an intermediate valence Mn. Besides this frequency for nanosized samples,... 

The copper atoms in the vast majority of the clusters can be assigned a formal charge of +1, while the chalcogen ligands are formally viewed as E or RE groups. Some of the selenium-bridged species, however - and nearly all copper telluride clusters - form nonstoichiometric compounds that display mixed valence metal centers in the formal oxidation states 0 and +I or +I and +11. These observations correlate with those made for the binary phases CU2S, Cu2 xSe, and Cu2- Te [38-40]. 

Again, clusters containing exclusively Cu, as well as compounds with mixed-valence metal centers, can be found. In contrast to the compounds presented in Schemes 3.3 and 3.4, one example of a selenido-selenolato-bridged cluster exists apparently containing Cu and Cu . Apart from this peculiarity, the cluster size and conformation of the Se -SeR-bridged compounds is now not only influenced by the nature and steric demand of the phosphine used, but it is also dependent on the spatial properties of the organic substituent at the selenium reactant This deter-... 

Figure 14.3 UV-vis-NIR spectroelectrochemistry of a mixed valence metal complex, reproduced from reference (20) with permission from The Royal Society of Chemistry.

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... 

Binary Phase Diagrams for Mixed Valency Metals... 

Micciche, F. vanStraten, M. A. Ming,W. Oostveen, E. VAN Haveren, J. van der Linde, R. Reeduk, J. Identification of mixed-valence metal clusters in drier solutions for alkyd-based paints by electrospray ionization mass spectrometiy (ESI-MS). Int. J. Mass Spectrom. 2005, 246(1-3), 80-83. 

Van Kuiken BE, Valiev M, Daifuku SL, Bannan C, Strader ML, Cho H, Huse N, Schoenlein RW, Govind N and Khalil M 2013 Simulating ru 13-edge x-ray absorption spectroscopy with time-dependent density functional theory Model complexes and electron localization in mixed-valence metal dimers. The Journal of Physical Chemistry A 117(21), 4444-4454. 

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