Atom cluster , metal

Herrmann A, Leutwyler S, Schumacher E and Woste L 1978 On metal-atom clusters IV. Photoionization thresholds and multiphoton ionization spectra of alkali-metal molecules Hel. Chim. Acta 61 453... 

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. 

Shi, X. and Zhang, H. (1992) Cluster rotamerism of a 25-metal-atom cluster [(Ph3P)ioAui3Agi2Br8] monocation a molecular rotary unit Journal of the Chemical Society, Chemical Communications, (17), 1195—1196 ... 

Living Colloidal Metal Particles from Solvated Metal Atoms Clustering of Metal Atoms in Organic Media... 

There is now not only a great number but also a great variety of metal atom cluster compounds. In this essay I should like to discuss the differences between those that have metal atoms in a relatively high mean oxidation state (+2 to +4, and even, in rare cases, a bit higher) and those with metal atoms in oxidation states in the range -1 to +1. To keep the discussion within reasonable limits I shall restrict it almost exclusively to clusters consisting of only two or three metal atoms. I eschew the pedantic assertion that two atoms do not a cluster make. 

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. 

The cluster 15, which crystallizes in the rhombohedral space group R3, has a remarkably different structure with respect to the structural features of 3a, 5a, and 10 (see Section II,D). Although a dode-cameric aggregate is present, as is the case for 5a (21), the cluster framework in 15 does not adopt a globular shape (39). The structure is built up of 24 Cu atoms surrounded by 12 triorganosilylphos-phanediyl moieties. The 24-metal-atom cluster consists of three planar Cu6 rings and two peripheral Cu3 cycles that all lie parallel to one another (Fig. 17). 

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. 

Five-metal-atom clusters have been obtained only with osmium. The binary carbonyl Os5(CO)i8 was initially formed in low yield (—10%) by the thermolysis of Os3(CO)12 (201). The yield of the compound may be... 

A metal atom cluster as defined by Cotton [1] is still a very broad term, because non-metal atoms can also be part of the cluster core. In this chapter mainly two types of metal atom clusters are presented the polyborane analogous polyhedral and the metalloid clusters E Rr of group 13 elements E. The structures and bonding of the polyhedral clusters with n < r are similar to those in the well-known polyboranes. 

Figure 6.40 Typical metal-atom clusters found in transition-metal carbonyls. For clarity, the carbonyl groups are not shown. Open circles denote carbon atoms in the cluster compound. (After Edwards Sienko, 1983.)...

Definitions of a metal atom cluster compound have been given (109, 233, 241, 316). Accordingly, in this review a cluster will be considered as a compound containing at least three metal atoms connected by metal-metal interactions in a triangular or polyhedral array. 

Much of the current literature on metal atom cluster species employs bonding concepts that are derived from MO treatment of the polyhedral borane anions, We thus begin by discussing these species, of which the most important examples are shown in Figure 8.15. We shall deal with the BaHg" ion in detail to illustrate the general approach to these systems. 

There are a number of other important boron cage compounds as well as structurally similar metal atom cluster molecules that are not closed polyhedra. Generally, these may be regarded as derived from closed polyhedra by removal of one or two vertices. The diagram below illustrates how removal of one or two vertices generates the so-called nido and arachno relatives of a closo octahedral structure. 

Thus, the size effects for catalytic reactions of metal atom clusters in a gas phase are manifested only in very small, essentially quantum clusters, which are in essence nonmetal particles. Another situation takes place in films, containing a set of nanoparticles immobilized at a surface or inside of a dielectric matrix. In this case the influence of M nanoparticle size on catalytic activity and structure of products formed is observed for considerably larger already classical particles of sizes from 2 ( 150 atoms) to 20-30 nm ( 105 atoms) [113, 114]. It is necessary to note that catalytic properties of M nanoparticles in composite systems are determined substantially by their interaction with a matrix, which depends on the size of particles. 

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