Metal clusters definition

There are no known examples of supported clusters dispersed in crystallo-graphically equivalent positions on a crystalline support. Thus, no structures have been determined by X-ray diffraction crystallography, and the best available methods for structure determination are various spectroscopies (with interpretations based on comparisons with spectra of known compoimds) and microscopy. The more nearly uniform the clusters and their bonding to a support, the more nearly definitive are the spectroscopic methods however, the uniformities of these samples are not easy to assess, and the best microscopic methods are limited by the smallness of the clusters and their tendency to be affected by the electron beam in a transmission electron microscope furthermore, most supported metal clusters are highly reactive and... 

A metal cluster can be considered as a polynuclear compound which contains at least one metal-metal bond. A better definition of cluster catalysis is a reaction in which at least one site of the cluster molecule is mechanistically necessary. Theoretically, homogeneous clusters should be capable of multiple-site catalysis. Many heterogeneous catalytic reactions require multiple-site catalysis and for these reasons discrete molecular metal clusters are often proposed as models of metal surfaces in the processes of chemisorption and catalysis. The use of carbonyl clusters as catalysts for hydrogenation reactions has been the subject of a number of papers, an important question actually being whether the cluster itself is the species responsible for the hydrogenation. Often the cluster is recovered from the catalytic reaction, or is the only species spectroscopically observed under catalytic conditions. These data have been taken as evidence for cluster catalysis. 

As underlined by Mingos and Wales (1990), according to Cotton and Walton (1982) the following statement can be considered as a first-approach working definition a metal cluster compound may be defined as a group of two or more metal atoms where direct and substantial metal—metal bonding is present. 

We do not yet know the structure of any of the metal clusters in the FeMo or FeV proteins. Nor do we know the arrangement of these clusters in the proteins. However, even with structural definition, which is in progress( -82), we will have to continue to apply the most powerful tools of physical bioinorganic chemistry to determine how hydrogen and substrates are handled by this remarkable enzyme. 

The efforts to rationalize the formulas and structures of Zintl ions and related species predated extensive definitive structural information on anionic post-transition metal clusters obtained by Corbett and his group in the 1970s [8, 9]. After enough such structural information on the bare post-transition metal clusters became available, the resemblance of their polyhedra to the known polyhedral boranes became apparent. For this reason, the simple Zintl-Klemm concept has been largely superseded by newer, more advanced descriptions of chemical bonding in such clusters, initially those applied to the polyhedral boranes. 

In this review, we have attempted to compile a comprehensive listing of all of the mixed-metal clusters that have been prepared. It has been necessary to adhere to a specific definition of a mixed-metal cluster, and the following two criteria have been set by us for detailed coverage in this review. 

If the octahedral and icosahedral 13-atom nickel clusters are representative, an inhomogeneous magnetic field is not a good tool for separating isomers of transition metal clusters. This result is reminiscent of the fact that geometry plays no strong role in the ionization potential variation of alkali clusters (80). However, many calculations must be made before we can be definitive here. 

The focus of this paper is experimental work on the chemical properties of neutral transition metal clusters. The outline is as follows. We first discuss in some detail the techniques used to generate neutral gas-phase clusters. Next the known physical properties of metal clusters are summarized. This is followed by a discussion of the definition of chemical reactivity in the context of the cluster experiments. Finally, several examples of specific reactions are presented and an electronic model is proposed which can explain many of the more striking observations. Results from recent cluster ion reaction studies... 

The past two and a half decades have witnessed a fast expanding chemistry of transition metal complexes containing more than one metal atom. Such complexes are generally referred to as transition metal clusters. Several definitions have been proposed for the term cluster, the one... 

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