Germanium-stabilized alkylation

In summary, the four chemical systems described in this paper demonstrate the versatility and selectivity of electrochemical methods for synthesis and characterization of metal-carbon a-bonded metalloporphyrins. The described rhodium and cobalt systems demonstrate significant differences with respect to their formation, stability and to some extend, reactivity of the low valent species. On the other hand, properties of the electroche-mically generated mono-alkyl or mono-aryl germanium and silicon systems are similar to each other. 

The crux of organic mechanistic stereochemistry may be the Walden inversion, the inversion of stereochemistry about a four-coordinate carbon atom by nucleophilic attack of, for example, a hydroxide ion on an alkyl halide. Many reactions of inorganic molecules follow the same mechanism. In contrast, the dissociative mechanism of tertiary halides to form tertiary carbocatanion intermediates is essentially unknown among the nonmetallic elements silicon, germanium, phosphorus, etc. The reason for this is the generally lower stability of species with coordination numbers of less than 4, together with an increased stability of five-coordinate intermediates. This difference is attributable to the presence of d orbitals in the heavier elements (Chapter 18). 

Many of the results reviewed here suggest that a replacement of the usual alkyl or aryl substituents by silyl substituents in unsaturated silicon and germanium compounds may be rewarding. As we noted, silyl substituents do tilt the properties of silylenes, silyl radicals, and sequential BDE trends toward those in carbon chemistry. They have already been shown to stabilize disilenes with respect to dissociation to two silylenes, and this may be crucial to the further development of digermene and distannene chemistry. 

Despite the greater thermal stability of the four-membered alkyl substituted cyclodisil-thianes, the cage compounds (RSi)4S6 are thought to possess the adamantane-like structure. The methyl derivative has such a structure (Si-S bond lengths 212.9 pm), Jike the germanium and tin derivatives36. 

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