Clusters, metallic nuclei

The action of Lewis bases on metal clusters leads to very many reactions. In the last section we analyzed a series of reactions in which the addition of bases induces different kind of oxidation-reduction processes. Nevertheless, Lewis bases can also induce other kind of reactions which do not imply a change in the formal oxidation state of the cluster metal nucleus. They are normally classified as ligand exchange or ligand substitution reactions. Specially interesting are those reactions in which the cluster structure remains unaltered in the substitution process. However, it should be also considered that ligand exchange... 

No details are available on the evolution of the four-iron butterfly cation to methane, but further protonation of the framework and reductive elimination of CH4 seem likely. The four-metal butterfly framework appears to play a significant role in these reactions, particularly in activating carbon monoxide through II —CO formation. Significantly, the proton-induced reduction has been observed with other four- and six-metal carbonyl clusters, but the reaction does not appear to occur with clusters with fewer nuclei (248). By analogy with the findings in the iron system, this minimum metal nucleus number requirement suggests that n —CO may be involved in all of these reactions. 

Let the chemical potential of a metal atom in the supersaturated state be uss and that in the saturated phase /Ueq- In the case of solutions, the supersaturation is given by ln(jCss/JCeq), where Xgs and Xeq are the mole fraction of metal atoms. In considering the fate of metal atoms in the overall phase change from fluid to solid state, it is clear that some of them end up in the bulk (interior) of the crystal nuclei, while others become part of the surface. Each atom in a cluster can potentially form six intermolecular interactions. In the bulk all these interactions are satisfied, while on the surface, the atoms are in a different energetic state because they cannot realize their full interaction potential. In a cluster or nucleus containing z atoms, Zb have the properties of a bulk solid and Zs are surface atoms. The free energy of the cluster, g, can be written as the sum of the bulk and surface free energies, gb and gs. ... 

Fig. 2.36. Examples of high nuclearity gold clusters (a) Metal nucleus in the clusters [AufAuijCUlPRaliol i (b) [Au(Au,Cl3PR3)6] ...

Because of the nature of metal clusters, assignation of oxidation numbers to individual metal atoms as it is usually made for mononuclear species does not make sense in the case of metal aggregates. Cluster polyatomic metal nucleus must be considered as a whole, so only average oxidation numbers or states can be assigned to individual atoms. 

Protonation and deprotonation reactions of carbonyl cluster species are in general rather slow. A screening effect of the carbonyl groups around the metal nucleus which makes the direct interaction of the reactants difficult is considered to be the cause of large activation barriers. 

Since metal clusters containing hydrogen directly bonded to the metallic nucleus are often involved in the synthesis, reactivity, and catalytic properties of cluster species, they have been frequently mentioned in the chemistry discussed in Chapter 2. Nonetheless, some features related to the structure of and bonding in these compounds will be discussed separately in this Section. 

As pointed out above, an STM tip can be used to nucleate and grow single clusters. In this type of experiment, cluster deposition on a STM tip is achieved when it is retracted about 10 to 20 run from the substrate surface. Under these conditions, where the feedback loop is disabled, absence of mechanical contact between the tip and the substrate in ensured. Then a positive potential pulse is applied to the tip, the metal deposited on it is dissolved, and it diffuses toward the substrate surface, where a nucleus develops and grows to yield a cluster, typically 20 nm wide. 

A metal surface that is uniformly flat offers no sites for further growth. In this case a new nucleus, or center of growth, must be formed. Since small clusters of metal atoms consist mainly of surface atoms, they have a high energy content, and their formation requires an extra energy. The basic principles of the formation of new nuclei can be understood within a simple model. We consider a small three-dimensional cluster of metal atoms on a flat surface of the same material, and suppose that the cluster keeps its geometrical shape while it is growing. A cluster of N atoms has a surface area of ... 

Aggregation of the atoms or microclusters may give metal nuclei. The micro-cluster itself may work as the nucleus. Although the size of microcluster or nucleus is not clear, the nucleus may consist of 13 atoms, which is the smallest magic number, This idea may be supported by the structural analysis of PVP-stabilized Pt nanoparticles (64) and other systems. In fact, a particle of 13 atoms is considered an elemental duster. In the case of preparation of PVP-stabilized Rh nanoparticle dispersions by alcohol reduction, formation of very tiny particles, the average diameter of which is estimated to be 0.8 nm, was observed (66). These tiny particles in the metastable state may consist of 13 atoms each and easily increase in size to the rather nanoparticles with average diameter of 1.4 nm, i.e., the particles composed of 55 atoms. This observation again supports the idea that the elemental cluster of 13 atoms is the nucleus. 

Growth of Nuclei to Metal Nanoparticles. If the elemental cluster of 13 atoms is the nucleus, the growth of nuclei to metal nanoparticles could proceed by deposition of atoms or microclusters on the surface of nuclei. This process is understandable based on the consideration of the formation of monodispersed nanoparticles. However, structural analysis has often proposed the aggregation of elemental clusters to form fundamental clusters (64). A similar idea is discussed for the structural analysis of bimetallic nanoparticles with cluster-in-cluster structure (40,61). 

---