Clusters silicide

Based on the above ideas on the reaction mechanism, it is expected that the silylene species at higher temperatures display an increased electron acceptor ability compared to the silicide cluster. Evidence for that is provided by investigations of catalyst samples using ferro-magnetic resonance (FMR). 

Scheme 1 demonstrates that different products result in dependence on the type of coimection between the CH3SiCl3 molecule and the silicide cluster on the catalyst surface. The preferred connection over two chlorine atoms is plausible and seems to be the reason for the kinetically controlled generation of dichloromethylsilane. 

Scheme 1. Different possibilities of connection between the CH3SiCl3 molecule and the silicide cluster...

The intermediate Cu3Si phase should be only one of some catalytical active siiicides under these conditions. The mechanism, which suggests the intermediate formation of surface fixed silylenes, requires that the silicide clusters alternately release and accept silicon atoms (see hydrodehalogenation). 

Region I is characterized by a metal catalyst VLS growth, as indicated by the metal caps on top of the nanostructures. The diameter of the nanowires in this growth process is determined by the diameter of the liquid alloy droplet at their tips. Metal silicide clusters of different sizes are present in the flowing gas above... 

In conclusion, the effect of ion bombardment on amorphous Cr-O-Si layers is manifested in major surface chemical and short-range structural changes. Ar bombardment can be used to create chromium silicide clusters. To avoid such ion-beam-induced transformations, depth profiling by wet chemical etching may be preferable. 

The product, Si4(SiBu 3)4, forms intensely orange crystals that are stable to heat, light, water and air, and do not melt below 350°. The Si-Si distances within the closo-Si4 cluster are 232-234 pm and the exo Si-Si distances are slightly longer, 235-237 pm (cf. Si-Si 235.17 in crystalline Si). Comparison with the closo-amon Si4 , which occurs in several metal silicides (p. 337) and is isoelectronic with the P4 molecule, is also appropriate. 

The sole example of a silicon-platinum cluster is the compound in entry 24 its structure has been noted in Section IV,A. It seems very likely that many further cluster systems await discovery, particularly with iridium, platinum, and gold, and that this represents an important future area of research. One obvious application is as precursors to metal silicides with high metal silicon ratios using c.v.d. techniques (compare Section V,A). 

The obtained results on the reaction mechanism can be summarized as follows The metal silicides form cluster structures which represent electron buffer systems. They can be oxidized or reduced easily by surface reactions. The adsorption of SiCl4 molecules at the cluster surface is immediately followed by an electron transfer from the cluster to the silicon atom of SiCl4, the cluster is oxidized. As a result of such a process a silylene species is formed at the surface of the catalyst. Chloride ions act as counter ions to the positive cluster, supporting the redox step (Eq. 4). 

The second example comes from solid-state chemistry, and the coimection with cluster chemistry is less obvious. Despite this, it is an excellent example of how E/M variation can lead to systematic variation in structure and, consequently, to properties. Although the example is taken from solid-state metal borides, the silicides see Silicon Inorganic Chemistry) and phosphides (see Phosphides Solid-state Chemistry) could have been used. 

Ag does not form a silicide or a carbide. Niles et al [105] studied Ag on ion-bombarded P-SiC using core-level and valence-band photoemission. Three-dimensional island growth is observed during deposition near 25 °C. The interface is non-reactive and exhibits ohmic behaviour. Annealing in the 400 - 600 °C range leads to coalescence of the Ag, with bare SiC between clusters, and higher temperatures lead to evaporation of essentially all the metal. 

Sn forms a silicide but not a carbide. Niles et al [106] have used core-level and valence-band XPS to study Sn on ion-bombarded P-SiC. The first monolayer grows as a uniform layer of a-Sn, while subsequent layers form clusters of P-Sn. Annealing in the 400- 1000°C range leads to out-diffusion of Si into the Sn layer with formation of Sn silicide. The reacted surface layer withstands anneals at as high as 1000°C without desorption of Sn. 

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