Cluster surface analogy

It has been argued that the behaviour of ligands on the edges and faces of metal clusters (molecular compounds based on three or more connected metals) might model the behaviour of such ligands respectively on a step or surface of a bulk metal (the cluster-surface analogy, Figure 2.19). 

Molecular transition metal clusters have long been proposed as models for some aspects of the chemistry at metal surfaces. Earl Muetterties was one of the first to promote this cluster-surface analogy... 

One of the driving forces behind organometallic cluster research is to model metal-surface catalysed processes such as the Fischer-Tropsch reaction. The cluster-surface analogy assumes that discrete organometallic clusters containing rf-block metal atoms are realistic models for the bulk metal. In many small clusters, the arrangements of the metal atoms mimic units from close-packed arrays, e.g. 

The cluster-surface analogy has been mainly used for adsorbates on heterogeneous metal surfaces in catalysis. Electrode metal surfaces also offer an opportunity to learn from the cluster-surface analogy. 

J2. The cluster-surface analogy surface organometallic chemistry J3. Supported clusters surfaces, zeolites J4. Metal clusters in catalysis... 

High-nuclearity clusters of the transition metals and a re-evaluation of the cluster-surface analogy Recent developments in the cluster chemistry of gold... 

Mimicking aspects of heterogeneous catalysis generating, isolating and reacting proposed surface intermediates on single crystals in vacuum Can we put the cluster-surface analogy on a sound structural basis ... 

Diagrams of hep, fee or bcc metal lattices such as we showed in Figure 6.2 imply flat metal surfaces. In practice, a surface contains imperfections such as those illustrated in Figure 27.14. The kinks on a metal surface are extremely important for catalytic activity, and their presence increases the rate of catalysis. In a close-packed lattice, sections of flat surface contain M3 triangles (27.25), while a step possesses a line of M4 butterflies (see Table 24.5), one of which is shown in blue in structure 27.26. Both can accommodate adsorbed species in sites which can be mimicked by discrete metal clusters. This has led to the cluster-surface analogy (see Section 27.9). 

An early stimulus to cluster chemistry was the cluster-surface analogy which proposed that cluster chemistry would resemble the surface chemistry of metals, because both surfaces and clusters consist of arrays of metal atoms. Supported metals such as Pd/C are very active catalysts. Clusters have so far not shown the high catalytic activity of either metal surfaces or mononuclear homogeneous catalysts, probably because clusters are poisoned by the pres-... 

In this last section we will discuss some of the reaction chemistry which has been observed with cluster compounds. Volumes could be written on this aspect of metal clusters alone. We will simply discuss the cluster-surface analogy as well as demonstrate the variety of reactions in which cluster compounds may participate. In doing so the complexity and difficulties of cluster chemistry will surely become apparent. 

---