The surface-cluster analogy

The chemistry of metal carbonyl clusters containing exposed or semi-exposed main-group elements has received attention because of the belief that such molecular clusters might serve as models for the chemisorption and subsequent reactivity of the corresponding species on a metal surfaceJ In particular, the M4 butterfly clusters containing / 4-carbido and -nitrido ligands have played a pivotal role in the development of the area, because of their obvious similarities to stepped-sites on metal surfaces. 

Nitrido clusters have also been proposed as models of intermediate species in many important catalytic reactions such as the Haber process and the reduction of NO c pollutants. Several surface reactions involving NO or N2 are known to occur via an adsorbed nitrogen atom in the Haber process the mechanism is believed to occur via formation of the metal nitride followed by successive addition of hydrogen atoms to the nitrogen, giving ammonia.t  

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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... 

The asymmetric unit contains one copy each of the subunits VPl, VP2, VP3, and VP4. VP4 is buried inside the shell and does not reach the surface. The arrangement of VPl, VP2, and VP3 on the surface of the capsid is shown in Figure 16.12a. These three different polypeptide chains build up the virus shell in a way that is analogous to that of the three different conformations A, C, and B of the same polypeptide chain in tomato bushy stunt virus. The viral coat assembles from 12 compact aggregates, or pen tamers, which contain five of each of the coat proteins. The contours of the outward-facing surfaces of the subunits give to each pentamer the shape of a molecular mountain the VPl subunits, which correspond to the A subunits in T = 3 plant viruses, cluster at the peak of the mountain VP2 and VP3 alternate around the foot and VP4 provides the foundation. The amino termini of the five VP3 subunits of the pentamer intertwine around the fivefold axis in the interior of the virion to form a p stmcture that stabilizes the pentamer and in addition interacts with VP4. 

In siunmary, although the application of detailed chemical kinetic modeling to heterogeneous reactions is possible, the effort needed is considerably more involved than in the gas-phase reactions. The thermochemistry of surfaces, clusters, and adsorbed species can be determined in a manner analogous to those associated with the gas-phase species. Similarly, rate parameters of heterogeneous elementary reactions can be estimated, via the application of the transition state theory, by determining the thermochemistry of saddle points on potential energy surfaces. 

This molecular picture of the surface states of chemisorbed molecules was consolidated by the analogy of the bonding of such molecules on molecular metal cluster and on small metal particles [11-13]. 

Scheme 2. Schematic relationships between C2 hydrocarbon adsorbates and the surface species that could be derived from them. Possible relationships are indicated between surface species that involve not more than one addition/subtraction of H and/or M atoms. Strong bonding of these species to metal atoms could give rise to structures approximating to metallocyclopropanes or metallocyclopropenes, respectively. These structures with C=C or C = C groupings frequently occur on surfaces with additional n bonds to further metal atoms, M in other cases, two additional CM bonds may replace a CC double bond. JOn surfaces the metal atoms are usually bonded to each other so that the analogous molecular cluster compounds would be metallocyclopropanes or metallocyclobutanes, etc. 1 See footnote to Scheme I. The dashed rectangles indicate surface species that involve no CH bond breaking on adsorption of the parent hydrocarbon. These are most likely to be present under low-temperature adsorption conditions.

Muetterties et al. (130) published an extensive survey relating the properties of metal clusters and their ligands to analogous properties of metal surfaces and adsorbed species. Moskovits (131) has expressed caution, particularly in relating the reactivities of cluster ligands and the surface equivalents and more recently,... 

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