Cluster complexes analogy with surfaces

The cluster approach is, like the extended-surface approach, characterized by many different calculational schemes. A recent review stresses electronic aspects of bonding. In this review we have chosen to concentrate on geometric aspects. We shall discuss a number of major techniques in order of increasing computational complexity the extended H uckel theory, self-consistent Xa scattered wave calculations, and self-consistent ab initio Hartree-Fock and valence bond methods. In that order these techniques allow increasing accuracy. However, the cluster size must decrease simultaneously due to calculational complexity, ultimately reducing the degree of analogy with surfaces. 

The surface reaction of impregnated mixed metal cluster complexes may be analogous to that of homometallic clusters on hydrated and dehydrated metal oxides as described in Sections III and IV. Bimetallic clusters are converted to anionic surface species by simple deprotonation via 0 on dehydrated MgO or AI2O3 surfaces these species have been characterized by IR spectroscopy (119). The ionic interaction with surface cations such as AF and Mg is demonstrated by IR and NMR measurements. The surface polynuclear carbonyl anions are stable up to about 373 K. If heated in vacuo at higher temperature, extensive decomposition takes place to give a mixture of Ru (or Os) metal particles and Fe oxides, accompanied by the evolution of H2, CO, and CO2. 

In the period reviewed here (1975-81), numerous papers dealing with the application of conventional spectroscopies to the surface chemistry of supported and/or grafted transition-metal ions have appeared. As in many cases the transition ions are isolated or present as very small clusters, the surface complexes formed on contact with suitable ligands (test molecules) show strong analogies with known complexes in homogeneous chemistry. 

Alternatively, supported metal carbonyl clusters are formed by surface-mediated synthesis" from mononuclear precursors. For example, [HIt4(CO), ] is formed from Ir(CO)2(acac) on MgO, and Rhg(CO)ig is formed from Rh(CO)2(acac) on MgO or y-AljOj in the presence of CO. Synthesis of metal carbonyl clusters on oxide supports apparently often involves OH groups or water on the support surface analogous chemistry occurs in solution. The synthesis from a mononuclear metal complex is likely to occur with a yield less than that associated with simple adsorption of a preformed metal cluster, and so performed cluster precursors are preferred, except when they do not fit into the pores of the support (e.g., a zeolite). 

Parenthetically, the discovery of 118, and of some other surface species bound in ways no molecular complex shows, should make inorganic and organometallic chemists read the surface literature not only to find references with which to decorate grant applications. The surface-cluster analogy, of course, is a two-way street. So far, it has been used largely to... 

Metal carbonyls have many derivatives obtained by (formal) substitution of a carbonyl for another ligand, including hydride. The interest in the complexes arose because they were considered as potential models for hydrogen on metal surfaces ( 7.3.1). In general this is valid, in that the frequencies fall in similar ranges to those found on metal surfaces, the detailed interaction with the surface is distinctly different from that in a transition metal cluster and the analogy is imperfect. 

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