Clusters, Nanoparticles, Materials, and Surfaces

We now go beyond mononuclear metal complexes to look at larger structures. Beginning with metal clusters, typically consisting of 2-50 metal atoms, we move to metal nanoparticles (NPs) with 50 to ca. 10 atoms and finally to bulk organometdlic materials. 

A cluster is a small fragment of metal, often surrounded by stabilizing ligands. Hiis picture is best adapted to understanding NPs, where a somewhat larger core is involved than in a cluster. More labile ligands than CO are often chosen for NPs to favor dissociation to reveal the surface metal atoms for enhanced reactivity, for example, when a substrate for a catalytic reaction is present. The term nanocluster is sometimes used for structures having - 50 atoms that are intermediate in size between clusters and NPs. 

The Organometallic Chemistry of the Transition Metals, Sixth Edition. Robert H. Crabtree. 

Reactive organometallic groups can be grafted to surfaces of materials, such as Si02, so as to give unusual reactivity patterns. We therefore conclude this survey with a study of some typical cases. 

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Cluster research is a very interdisciplinary activity. Teclmiques and concepts from several other fields have been applied to clusters, such as atomic and condensed matter physics, chemistry, materials science, surface science and even nuclear physics. Wlrile the dividing line between clusters and nanoparticles is by no means well defined, typically, nanoparticles refer to species which are passivated and made in bulk fonn. In contrast, clusters refer to unstable species which are made and studied in the gas phase. Research into the latter is discussed in the current chapter. 

Nanoparticle materials are important because they exhibit unique properties due to size effects, quantum tunneling, and quantum confinement. As sizes of embedded particles are reduced to the nanometer scale, the surface-to-bulk ratio increases significantly. Therefore, surface effects can dominate bulk properties and an understanding of nanosurfaces becomes important. In this chapter, we discuss characterization of vacancy clusters that reside on surfaces of embedded nanoparticles as well as studies on the correlation of surface vacancy clusters to the properties of the nanomaterials. 

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