Metal atom cluster compounds

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. 

N.N. Greenwood Whether your compounds are described as hyper-closo or iso-closo depends on the number of electrons assumed to be contributed by the metal atom to the cluster. If, as is generally assumed, ruthenium contributes two electrons to the cluster in compounds such as yours, then RUC2B7 has a closo 22e skeltal electron count (i.e. 2n+2) rather than a 20e hyper-closo count. The uncertainty concerning the most appropriate choice of formal oxidation state for metals in covalent compounds permeates... 

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... 

Synthesis of nano-structured alloys by the inert gas evaporation technique A precursor material, either a single metal or a compound, is evaporated at low temperature, producing atom clusters through homogeneous condensation via collisions with gas atoms in the proximity of a cold collection surface. To avoid cluster coalescence, the clusters are removed from the deposition region by natural gas convection or forced gas flow. A similar technique is sputtering (ejection of atoms or clusters by an accelerated focused beam of an inert gas, see 6.9.3). 

The material reviewed in this Chapter hitherto has focused on metallacarboranes in which the metal atom is a vertex in an icosahedral cage framework. Until recently, monocarbollide metal compounds with core structures other than 12 vertexes were very rare since suitable carborane precursors were not readily available." However, Brellochs recent development of the reaction of decaborane with aldehydes to give 10-vertex monocarboranes permits a considerable expansion in this area of boron cluster chemistry. As a consequence, several intermediate-sized monocarboranes are now easily accessible and we have recently begun to exploit the opportunities that these present. In particular, we have focused thus far on complexes derived from the C-phenyl-substituted species [6-Ph- zJo-6-CBgHii] It is clear from these initial studies that a wealth of new chemistry remains to be discovered in this area, not only from among the metal derivatives of PhCBg car-boranes such as those discussed in this section, but also in the metal complexes of other newly available carboranes. 

This is one of two articles in this volume concerned with the borane-carborane structural pattern. In the other (see Williams, this volume, p. 67) Williams has shown how the pattern reflects the coordination number preferences of the various atoms involved. The purpose of the present article is to note some bonding implications of the pattern, and to show its relevance to a wide range of other compounds, including metal clusters, metal-hydrocarbon n complexes, and various neutral or charged hydrocarbons. 

In the cellular multiple scattering model , finite clusters of atoms are subjected to condensed matter boundary conditions in such a manner that a continuous spectrum is allowed. They are therefore not molecular calculations. An X type of exchange was used to create a local potential and different potentials for up and down spin-states could be constructed. For uranium pnictides and chalcogenides compounds the clusters were of 8 atoms (4 metal, 4 non-metal). The local density of states was calculated directly from the imaginary part of the Green function. The major features of the results are ... 

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.)...

Analysis of the valence-band spectrum of NiO helped to understand the electronic structure of transition-metal compounds. It is to be noted that th.e crystal-field theory cannot explain the features over the entire valence-band region of NiO. It therefore becomes necessary to explicitly take into account the ligand(02p)-metal (Ni3d) hybridization and the intra-atomic Coulomb interaction, 11, in order to satisfactorily explain the spectral features. This has been done by approximating bulk NiO by a cluster (NiOg) ". The ground-state wave function Tg of this cluster is given by,... 

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