Mixed-Valence Metal Complexes

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... 

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]. 

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)... 

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. 

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.

The eight-coordinate vanadium complex V(S2CMe)4 contains both dodecahedral and square prismatic eight-coordinate molecules in the same crystal.322 Of particular interest is the chain-like, mixed valence platinum complex [Pt2(S2CMe)4( i-I)]A, which displays unusual electrical properties metallic conduction between 300 and 340 K and semiconducting properties below 300 K,323 whereas the analogous nickel complex, [Ni2(S2CMe)4(p-I)]x is a semiconductor.324... 

Mixed-valence metal string complexes of gold... 

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... 

Mixed valence glyoximato-complexes of Pd and Pt have been obtained as diamagnetic black crystals with a metallic lustre by treatment of the glyoximato-metal complex with iodine in dichlorobenzene. The A -ray structure shows that on forming the mixed valence compound, [(C2H3N202)2PdI], the Pd—Pd distance is shortened from 3.558 to 3.244 A. It has also been shown that the metal complexes of Pd, Pt, with... 

Ir2 systems may also become active toward H2 addition via disproportionation. The reaction sequence depicted in Scheme 8 illustrates how the transformation of an Ir2 symmetric structure (d -d system) into its related mixed-valence It2° ° complex (d -d system) results in the generation of an empty orbital at the Ir(II) (d metal center) that permits H2 coordination and subsequent oxidative additirai [44, 46]. 

Photoswitching of the electronic communication between two metal centers using the photochromic diarylethene bridging ligand with both thiophene moieties substituted with two cyclometallated phenylpyridines (Figure 23a) was first reported by Coudret and Launay. Although the comproportionation constants (Kc) for the dinuclear mixed-valence ruthenium complexes in both the open and closed forms were similar, the intervalence charge-transfer (IVCT) transitions were observed only in the closed... 

Fig.10. Schematic view of the zero-order (dotted curves) and first order (continuous curves) states for a symmetrical mixed-valence trinuclear complex of the type Ru(ll)-Ru(lll)-Ru(ll), assuming electronic couplings of 2000 and 1000 cm between adjacent and remote metal centers, respectively.

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... 

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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