Atom cluster , metal chemistry

Cotton, F. A. (1965). Metal-Metal Bonding in [Re2X8]2- Ions and Other Metal Atom Clusters. Inorganic Chemistry,... 

Low-temperature, photoaggregation techniques employing ultraviolet-visible absorption spectroscopy have also been used to evaluate extinction coefficients relative to silver atoms for diatomic and triatomic silver in Ar and Kr matrices at 10-12 K 149). Such data are of fundamental importance in quantitative studies of the chemistry and photochemistry of metal-atom clusters and in the analysis of metal-atom recombination-kinetics. In essence, simple, mass-balance considerations in a photoaggregation experiment lead to the following expression, which relates the decrease in an atomic absorption to increases in diatomic and triatomic absorptions in terms of the appropriate extinction coefficients. 

The Characterization and Properties of Small Metal Particles. Y. Takasu and A. M. Bradshaw, Surf. Defect. Prop. Solids p. 401 1978). 2. Cluster Model Theory. R. P. Messmer, in "The Nature of the Chemisorption Bond G. Ertl and T. Rhodin, eds. North-Holland Publ., Amsterdam, 1978. 3. Clusters and Surfaces. E. L. Muetterties, T. N. Rhodin, E. Band, C. F. Brucker, and W. R. Pretzer, Cornell National Science Center, Ithaca, New York, 1978. 4. Determination of the Properties of Single Atom and Multiple Atom Clusters. J. F. Hamilton, in "Chemical Experimentation Under Extreme Conditions (B. W. Rossiter, ed.) (Series, "Physical Methods of Organic Chemistry ), Wiley (Interscience), New York (1978). 

Metal polysulfido complexes have attracted much interest not only from the viewpoint of fundamental chemistry but also because of their potential for applications. Various types of metal polysulfido complexes have been reported as shown in Fig. 1. The diversity of the structures results from the nature of sulfur atoms which can adopt a variety of coordination environments (mainly two- and three-coordination) and form catenated structures with various chain lengths. On the other hand, transition metal polysulfides have attracted interest as catalysts and intermediates in enzymatic processes and in catalytic reactions of industrial importance such as the desulfurization of oil and coal. In addition, there has been much interest in the use of metal polysulfido complexes as precursors for metal-sulfur clusters. The chemistry of metal polysulfido complexes has been studied extensively, and many reviews have been published [1-10]. 

The existence of two classes of metal atom cluster compounds is a fact of Nature. Like many such facts it is not neatly delineated there are many blurred boundaries, few quantitative relationships, and exceptions to most if not all generalizations concerning it. Despite this, the way we recognize the difference, use it, and try to account for it is a good example of why chemistry is both less exact and more interesting (to me) than physics and mathematics. We chemists are forced to tackle far more complex and "messy" problems than workers in these other fields and, in our own way, I think we make a good job of it. 

As the main theme of this meeting is to assess and consolidate past achievements in various key areas of inorganic/organo-metallic chemistry, with the objective of gazing deep and hard into the futuristic chemical crystal ball of the 21st century, the purpose of my presentation will be to focus attention on pivotal developments in the field of transition metal atom/metal cluster chemistry over the past decade and then to attempt to project and forecast some of the more promising directions that the area is likely to follow in the years ahead. 

Even though qualitative bonding descriptions of metal atom clusters up to six or seven atoms can be derived and in some cases correlated with structural detail, it is clear that most structures observed for higher clusters cannot be treated thus. Nor do the structures observed correlate with those observed for borane derivatives with the same number of vertices. Much of borane chemistry is dominated by the tendency to form structures derived from the icosahedron found in elemental boron. However, elemental transition metals possess either a close-packed or body-centered cubic arrangement. In this connection, one can find the vast majority of metal polyhedra in carbonyl cluster compounds within close-packed geometries, particularly hexagonal close-packing. 

An additional significance for carbidocarbonyl clusters has appeared more recently with the discovery of the fascinating reactivity of carbon atoms in clusters when they are exposed to reactive molecules in low nuclearity carbidocarbonyl clusters. These observations followed on the heels of the recognition of the crucial role played by surface bound carbon atoms in metal-catalyzed carbon monoxide hydrogenation, and so a new area of overlap between cluster chemistry and surface chemistry has arisen. Moreover, in this case the comparisons between organometallic and surface chemistry may lie in reactivity and not merely structural similarities. 

This methodology, which allows the measurement of covalent bond energies, has been applied to a wide variety of systems. Many of these have been reviewed elsewhere and include atomic transition metal cations bonded to H, C, N, O, CH, NH, OH, CH2, NH2, CH3 [29,93], S, and 2S [99] transition metal cluster cations bound to D, O, [100], S [101], CH, CH2, and CH3 [102] and various inorganic species relevant to plasma chemistry [103]. 

It was in 1963, with yet another accidental discovery, that of the [RegCl ]3 ion (6, 7) that the field of "metal atom cluster" chemistry really had its birth, since this discovery provoked the first general discussion of the existence and probable importance of the entire class of "metal atom cluster" compounds (6). 

The field of metal atom clusters and M-M multiple bonds is no island separated from the rest of chemistry, but is related as a peninsula to the mainland. It, therefore, has considerable relevance and, indeed, importance to the rest of chemistry. 

Concluding Statement. Over the past years there has arisen a whole new transition metal chemistry, that of metal atom clusters and metal-metal multiple bonds. This is conceptually as well as historically a new and revolutionary departure from the preexisting Werner type chemistry, and has one aspect, namely, the quadruple bond, that is a totally new concept in chemistry as a whole. The activity and progress in this field over the past 17 years has been phenomenal and shows every sign of continuing unabated for some time. Very probably important technical applications of compounds with M-M bonds will soon become more numerous. 

Since this concept is a relatively familiar one, let us simply provide a few examples that are pertinent to metal atom cluster chemistry and organometallic chemistry. Some of them derive directly from the isoelectronic relationships already noted in connection with simple metal carbonyl molecules. 

These two elements have very similar chemistries, though not so nearly identical as in the case of zirconium and hafnium. They have very little cationic behavior, but they form many complexes in oxidation states II, III, IV, and V. In oxidation states II and III M—M bonds are fairly common and in addition there are numerous compounds in lower oxidation states where metal atom clusters exist. An overview of oxidation states and stereochemistry (excluding the cluster compounds) is presented in Table 18-B-l. In discussing these elements it will be convenient to discuss some aspects (e.g., oxygen compounds, halides, and clusters) as classes without regard to oxidation state, while the complexes are more conveniently treated according to oxidation state. 

In large part, the reactivity of a metal cluster is the same as that of the individual metal fragments. The unique features of cluster reaction chemistry involve the bridging of ligands between two or more metal atoms, ligand migration between metal atoms, and metal-metal bond cleavage and formation. 

Metal Clusters in Chemistry, ed. P. Braunstein, L. A. Oro and P. R. Raithby, Wiley-VCH Verlag GmbH, Weinheim, Germany, 1999 R150 B. T. Heaton, J. A. Iggo, I. S. Podkorjdov, D. J. Smawfield and S. P. Tunik, Multinuclear NMR Studies on Homo- and Heterometallic Rhodium Clusters Containing 6 or More Metal Atoms , p. 960... 

The formation of polyhedral cages and clusters has only recently been recognized as an important and widespread phenomenon, but discoveries in this field have been frequent in the last decade and examples may now be found in nearly all parts of the Periodic Table. In this Section each of the principal polyhedra will be mentioned and illustrations given. Further details may be found under the chemistry of the particular elements and in the sections on metal-atom clusters (Sect. 19-11) and polynuclear metal carbonyls (Sect. 22-3). 

Kepert, D. L. and K. Vrieze, in Halogen Chemistry, Vol. 3, V. Gutmann, ed., Academic Press, 1967, p. 1 (review of metal-atom clusters containing M6X12, M6Xa, M3X9 and other groupings). 

Penfold, B. R., in Perspectives in Structural Chemistry, Vol. 2, J. D. Dunitz and J. A. Ibers, eds., Wiley, 1968 (encyclopedic review of metal-atom cluster and cage structures). 

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