Bonding in metal clusters

As discussed in the last section, the metal network in clusters may be considered, especially for high nuclearity clusters, as a finite portion of a compact metal structure stabilized by external ligands. The feature that the number of atoms is relatively small does not make the description of the system simpler. On the contrary, for pure metals as well as for traditional mononuclear compounds, there are theories which allow us to produce descriptions of their properties which, however, do not fit for clusters. In the very short history of this new class of compounds many efforts have been made to obtain some generalizations that permit us to rationalize experimental features as being structures and reactivity, as well as to perform quantitative theoretical calculations. The most important approaches to bonding in metal cluster are summarized in Table 2.8. 

Since cluster compounds in general are rather complex species the application of quantitative methods for describing bonding is not only difficult but also impractical. Qualitative approaches and empiric rules often play an important role in this case. Simple methods permitting the systematization and projection of known features are very valuable for practical chemists. 

The total number of cluster valence electrons (i.e. metal valence electrons plus those apported by the ligands) is such that each main group atom as well as each transition metal atom in the cluster has 8 and 18 electrons respectively. Clusters which follow this rule are known as electron precise species. 

Bonding pattern in metal clusters is analogous to that in the boranes (electron defficient species) where each atom contributes with 3 orbitals and 2 electrons to the skeletal bonding. 

The addition of secuensive pairs of electrons to a c/050-cluster opens its structure originating nido and arachno structures, i.e. producing the remotion of cluster vertices (decapping). 

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Terminal M-H bonds in metal cluster complexes are occasionally found (see Table 3). Some of them are noteworthy in that they contain both terminal and bridging H ligands on the same metal atom. Examples include H2Os3(CO)n 78>... 

Gronbeck H., (1996) On the structure of and bonding in metal clusters thesis Gothenburg University and Chalmers University of Technolology, Gothenburg, Sweden and references therein... 

Theoretical work over the last 15 years has done much to advance our understanding of structure and bonding in metal clusters. The isolobal analogy " ... 

Fig. 6.23 Tungsten-tungsten bond lengths as a function of bond order. Each of the points represents the mean and range of several values. See Chapter 16 for discussion of multiple bonds in metal clusters.

Analysis of chemical bond in metal clusters I. Alkali metal and alkaline earth metals... 

Analysis of Chemical Bond in Metal Clusters 1. Alkali Metal and Alkaline Earth Metals 239... 

As the field of metal carbonyl cluster chemistry was expanding rapidly many other new classes of clusters joined the fold, in particular, gold phosphine clusters, metal-alkoxide, -thiolate, and -halide clusters of the early transition elements. The theories of bonding in metal clusters rapidly advanced aided by both computational procedures and conceptual ideas. Without a doubt the most significant in the latter... 

G3. Reactions of metal-metal bonds in metal clusters G4. Chemistry of metal clusters... 

If we consider the strength of the metal-metal bond in metal clusters and metals or crystallites, requisite data for a complete comparison of metal-metal bond strength are not fully available [100]. 

Metal Clusters. The 18-electron rule as an approach to the bonding in metal cluster supposes the skeletal atoms are held together by a network of localized bonds and that each individual cluster atom utilizes its nine atomic orbitals to form metal-metal bonds as well as to accommodate both metal valence electrons and ligand electron pairs. Consequently, in the application of the 18-electron rule to clusters, the following three assumptions should be taken into account ... 

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