Metallic clusters results

Heteronuclear compounds containing gold(I) and other metal atoms which present Au -M interactions are well represented in the area of metal carbonyl clusters. The addition of a AuPR3+ or Au2(/u-P-P)2+ fragment to a metal cluster results in the formation of Au—M bonds often with retention of the cluster framework. Several reviews have been reported recently,3153-3155 and so it will not be treated here. Some representative examples are found in Figure 26. 

Fig. 1 The effect of size on metals. Whereas bulk metal and metal nanoparticles have a continuous band of energy levels, the limited number of atoms in metal clusters results in discrete energy levels, allowing interaction with light by electronic transitions between energy levels. Metal clusters bridge the gap between single atoms and nanoparticles. Even though in the figure the energy levels are denoted as singlets, we must remark that the spin state of the silver clusters is not yet firmly established...

Sometimes, reactions in which an alcohol is oxidized by hydride transfer to a metallic cluster, resulting in the formation of a metallic hydride that subsequently transfers a hydride to a sacrificial aldehyde or ketone, are described as Oppenauer oxidations.44 In the opinion of the authors, the name Oppenauer oxidation should be reserved for oxidation of alcohols in which a hydride is directly transferred from a metallic alkoxide to an aldehyde or ketone acting as oxidant. 

In the sections below we will describe in detail the known photochemistry of di-, tri-, and tetranuclear metal clusters. Results will be discussed in simple electronic structural terms. 

Pandey, P. K. K., and Schatz, G. C. (1984) A detailed analysis of the Raman enhancement mechanisms associated with the interaction of a Raman scatterer with a resonant metal cluster Results for Lin-H2,y. Chem. Phys., 80, 2959-2972. 

The predominant species observed in SIMS spectra are singly charged atomic and molecular ions [51], However, inorganic and organic cluster ions can also be formed. If the sample consists of a simple single-component metal, then clusters such as M, M, etc., are observed in addition to M+ [52], Oxidation of the metal results in formation of MO ", MO/, M Oll", etc. The relative yield of MO+ to M+ depends on the bond dissociation energy of the oxide [52], For a two-component, oxidized metal, clusters of the type M/", M N, MjO, and M N O/ are observed [51]. 

As a result of the systematic application of coordination-chemistry principles, dozens of previously unsuspected stnicture types have been synthesized in which polyhedral boranes or their anions can be considered to act as ligands which donate electron density to metal centres, thereby forming novel metallaboranc elusters, ". Some 40 metals have been found to act as acceptors in this way (see also p. 178). The ideas have been particularly helpful m emphasizing the close interconnection between several previously separated branches of chemistry, notably boron hydride clu.ster chemistry, metallaboranc and metallacarbaborane chemistry (pp. 189-95). organometallic chemistry and metal-metal cluster chemistry. All are now seen to be parts of a coherent whole. 

Recent results in the chemistry of transition metal clusters with organic ligands. H. Vahrenkamp, Struct. Bonding (Berlin), 1977, 32, 2-56 (408). 

Pauling, L. Structure of Transition-Metal Cluster Compounds Use of an Additional Orbital Resulting from the f,g Character of spd Bond Orbitals Proc. Natl. Acad. Sci. (USA) 1977, 74, 5235-5238. 

A method has recently been described for wrapping polymers around metal atoms and very small metal clusters using both matrix and macroscale metal vapor-fluid polymer synthetic techniques. Significant early observations are that (i) the experiments can be entirely conducted at, or close to room temperature, (ii) the resulting "pol5aner stabilized metal cluster combinations are homogeneous liquids which are stable at or near room temperature, and (,iii) the methodology is easily extended to bimetallic and trimetallic polymer combinations. ... 

Vahrenkamp H (1977) Recent Results in the Chemistry of Transition Metal Clusters with Organic Ligands. 32 1-56... 

Research into cluster catalysis has been driven by both intrinsic interest and utilitarian potential. Catalysis involving "very mixed -metal clusters is of particular interest as many established heterogeneously catalyzed processes couple mid and late transition metals (e.g., hydrodesulfurization and petroleum reforming). Attempts to model catalytic transformations arc summarized in Section II.F.I., while the use of "very mixed -metal clusters as homogeneous and heterogeneous catalysis precursors are discussed in Sections I1.F.2. and I1.F.3., respectively. The general area of mixed-metal cluster catalysis has been summarized in excellent reviews by Braunstein and Rose while the tabulated results are intended to be comprehensive in scope, the discussion below focuses on the more recent results. 

Figure 7. Time-of-flight mass spectra showing results of platinun cluster reactions with benzene. The lower trace is clean metal without reactant. The upper trace is with the pulsed addition of. 21 % benzene in heliun. The notation indicates the nmber of adducts on each metal cluster. The metal cluster are all two photon ionized, while the observed products are single photon ionized, hence the enhancement of the product over metal signals. Reproduced from Ref. 17.

The EXAFS results suggested that the iridium-rhodium clusters dispersed on alumina differed in size and/or shape from those dispersed on silica, based on the result that the total coordination nunbers of the iridium and rhodium atoms in the clusters were very different (7 and 5 in the alumina supported clusters vs. 11 and 10 in the silica supported clusters). These coordination numbers suggested that the clusters dispersed on alumina were smaller or that they were present in the form of thin rafts or patches on the support. The possibility of a "raft-like" structure in the case of the alumina supported clusters suggests an interaction between the metal clusters and the support which is much more pronounced for alumina than for silica. If the clusters on the alumina were present as rafts with a thickness of one atomic layer, one could have a situation in which the rhodium concentration at the perimeter of the raft was greater... 

The extended fine structure (EXAFS) was used to determine bond distances, coordination number and disorder. The near edge (XANES) was used as an Indication of electronic state. Significant results Include, 1) a reversible change of shape of clean supported metal clusters as a function of temperature, 2) supported Pt clusters have more disorder or strain compared to the bulk metal, and 3) a clear determination of the bonds between the catalytic metal atoms and the oxygen atoms of the support. 

We have shown that the Fe or Fe-Co/Zeollte systems with a vs H/T type results provide a unique probe for obtaining particle sizes (3) of the metal clusters, and can be extended for computing a particle-size distribution. In addition, electronic interactions between the Bronsted acid sites and the Fe species can be elucidated. 

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