Silver clusters acceptor

Interestingly, non-metallic silver clusters, depending on their sizes, may act either as electron donors or as electron acceptors. Using sulfonatopropyl-viologen, SPV (E° for SPV/SPV- = — 0.41 V/NHE), pulse radiolysis established that small silver clusters (n < 4) acted as electron donors (Le. E° for (Ag /Agn < E° for SPV/SPV - ) while, conversely, large silver clusters (n 2 4) were electron acceptors (i.e. E° for Ag/ /Agn > E° for SPV/SPV- ) [511]. Size-dependent electrochemical potentials of silver aggregates have been elucidated (Fig. 82) [506]. 

An analogous mechanism is proposed for the activation of molecular oxygen and the cooperative binding of two oxygen molecules on the anionic silver clusters in which the first adsorbed O2 serves as an activator [351]. Since anionic silver clusters have generally lower VDE values than gold clusters, weaker electron acceptors such as O2 can already induce electron transfer and activate them, which is not possible in the case of Au . The first oxygen... 

Cluster properties, mostly those that control electron transfer processes such as the redox potential in solution, are markedly dependent on their nuclearity. Therefore, clusters of the same metal may behave as electron donor or as electron acceptor, depending on their size. Pulse radiolysis associated with time-resolved optical absorption spectroscopy is used to generate isolated metal atoms and to observe transitorily the subsequent clusters of progressive nuclearity yielded by coalescence. Applied to silver clusters, the kinetic study of the competition of coalescence with reactions in the presence of added reactants of variable redox potential allows us to describe the autocatalytic processes of growth or corrosion of the clusters by electron transfer. The results provide the size dependence of the redox potential of some metal clusters. The influence of the environment (surfactant, ligand, or support) and the role of electron relay of metal clusters in electron transfer catalysis are discussed. 

The aim of this work is to extend the kinetics study of electron transfer to monitor donors of more positive redox potential than previously studied, toward silver clusters, Ag ", as acceptors and thus to approach the domain where clusters get metal-hke properties (13). The selected donor is the naphta-zarin hydroquinone, with properties similar to those of the hydroquinone used as a developer in photography. Its redox potential depends on pH, so that different monitor potentials are available through control of pH. Moreover, the reactivity of the donor may be followed by variation of absorbance when naphtazarin hydroquinone, almost transparent in the visible, is replaced by oxidized quinone with an intense absorption band. 

The redox potentials of short-lived silver clusters have been determined through kinetics methods using reference systems. Depending on their nuclearity, the clusters change behavior from electron donor to electron acceptor, the threshold being controlled by the reference system potential. Bielectronic systems are often used as electron donors in chemistry. When the process is controlled by critical conditions as for clusters, the successive steps of monoelectronic transfer (and not the overall potential), of which only one determines the threshold of autocatalytical electron transfer (or of development) must be separately considered. The present results provide the nuclearity dependence of the silver cluster redox potential in solution close to the transition between the mesoscopic phase and the bulk metal-like phase. A comparison with other literature data allows emphasis on the influence of strong interaction of the environment (surfactant, ligand, or support) on the cluster redox potential and kinetics. Rela-... 

Many carbonyl and carbonyl metallate complexes of the second and third row, in low oxidation states, are basic in nature and, for this reason, adequate intermediates for the formation of metal— metal bonds of a donor-acceptor nature. Furthermore, the structural similarity and isolobal relationship between the proton and group 11 cations has lead to the synthesis of a high number of cluster complexes with silver—metal bonds.1534"1535 Thus, silver(I) binds to ruthenium,15 1556 osmium,1557-1560 rhodium,1561,1562 iron,1563-1572 cobalt,1573 chromium, molybdenum, or tungsten,1574-1576 rhe-nium, niobium or tantalum, or nickel. Some examples are shown in Figure 17. 

In 1943, Hieber and Lagally reported that the reaction of anhydrous rhodium trichloride with carbon monoxide at 80°C, under pressure, and in the presence of silver and copper as halogen acceptors, gave a black crystalline product which, on the basis of elemental analysis, was formulated as Rh4(CO)n 75). The exact nature of this compound was established 20 years later by Dahl using three-dimensional X-ray analysis which led to its reformulation as Rh6(CO)i6 53). This discovery can be regarded as the birthday of the chemistry of high nuclearity clusters. 

The hexanuclear carbonyl Rh6(CO)ie was first prepared by treating anhydrous rhodium trichloride with carbon monoxide at 200 atm for several hours in the presence of a halogen acceptor such as cadmium, copper powder, silver, or zinc at temperatures of 80-230°.i At temperatures of 50-80°, the main product was Rh4(CO)i2. Optimum yields (80-90 %) of Rh6(CO)i6 are obtained by allowing methanolic solutions of rhodium trichloride trihydrate to react with carbon monoxide at 40 atm and 60°. Recently, however, there have been reports - of new high-yield syntheses of rhodium cluster carbonyls which require only ambient pressures of carbon monoxide. Chini and Martinengo have obtained Rh6(CO)i6 and Rh4(CO)i2 in high yield from the reaction of Rh2(CO)4Cl2 with carbon monoxide at atmospheric pressure and room temperature. 

A charged cluster may constitute an electron acceptor, but that depends on its own redox potential value, E (A -Agn) relative to the threshold imposed by the monitor potential, E°(Q -QH2). As the redox potential increases with cluster nuclearity (5, 6), a certain time after the pulse is required to allow the first supercritical clusters to be formed and their potential to reach the threshold value imposed by the hydroquinone. When time, t, is less than tc, where n < Uc, the transfer is not allowed. During this induction period, the kinetics at 380 nm correspond to pure coalescence of clusters (Figure 4), and hydroquinone is stable (the bleaching OD512 is constant). That means, obviously, that none of the silver species present at that time can react with hydroquinone, especially free Ag ions and Ag ions associated with the smallest clusters. 

The metal will react readily with oxygen, chlorine, or fluorine to yield the respective products. The metal will also react slowly with nitric acid to give OSO4. Osmium(O) compounds involve several carbonyls, as well as phosphine, amine, arene, diene complexes, and the well-studied osmium clusters. The compounds are usually prepared by reducing an osmium salt in the presence of the appropriate ligand and, if necessary, a halide acceptor such as copper, silver, or zinc salts. 

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