Silicon germanium compounds

Multinuclear ( H, 13C, nB) NMR spectroscopy is undoubtly the most useful and powerful method for the investigation of the organoboration of 1-alkynyltin, -germanium, and -silicon compounds allowing not only the structure of the final products to be determined but also the product ratio to be ascertained (Tables 6-8). 

The resistance of the E14-S bond in cyclotrimetallathianes toward nucleophilic reagents, for example, water and alcohol,62 increases on going from the silicon compounds to the corresponding germanium and tin derivatives. This is due, most likely, to the fact that the reaction of less nucleophilic ylides with phenyl groups at the phosphorus atom with trithianes (R2MS)3 (M = Ge, Sn) occurs slowly and is impeded by several side processes. 

Quite recently, it was established that the formation of the pentacoordinated germanium and tin compounds EF4(CH2CH=CH2) from EF3(CH2CH=CH2) (E = Ge, Sn) by the addition of F is exothermic148. The nucleophilicity of the allylic y-carbon is much enhanced when the pentacoordinated EF4(CH2CH=CH2) is formed. These results are qualitatively similar to those found for the reaction of the corresponding silicon compound. The pentacoordinated Ge and Sn complexes have a significant Lewis acidity which allows them to form stable hexacoordinated intermediates in the course of the reaction. 

Most of the reactions of silicon or germanium organic compounds proceed photochemically via a radical mechanism or via a cycloaddition mechanism (Chapters 4 and 6). There are few examples of nucleophilic addition of RSi or RGe to Cgo [118,119]. Reaction of silyUithium derivatives RjSili or germyllithium derivatives RjGeLi with different alkyl- and aryl-substituents R yields mainly the 1,2-adduct 28 or the 1,16-adduct 29 1,4-addition and dimerization of two fullerene-units was also found as a minor pathway. One example is given in Scheme 3.15. 

The silicon-bismuth bond has been demonstrated in a number of compounds such as Bi(SiF3)3, a volatile crystalline compound184 which decomposes on heating it is air and moisture sensitive. Germanium-bismuth compounds can be prepared by the use of Bi(SiEt3)3 (equations 52 and 53).185... 

Again, the germanium analog of the product from reaction (154) can gain or lose CO to form an open germanetriyl or a closed tricobalt cluster derivative, but the silicon compound apparently undergoes neither of these reactions (46, 178). 

From subsequent work, however, it appears that the product isolated was ClCCo3(CO)9 278). Silicon compounds of the type RSiCo CO) corresponding to one of the germanium-cobalt products of reactions (156), have not yet been made neither has an analog of the spiroderiva-tive Ge[Co2(CO)7]2 191). Synthetic efforts directed towards these systems would be valuable. 

FIGURE 37. Comparison of calculated and experimental 73Ge NMR chemical shifts of some simple germanium compounds. The experimental values of GeMe4 nCln (n = 1-3) were not available. They have been estimated from the analogues silicon compounds using empirical correlation factors. Reproduced by permission of John Wiley Sons, Inc. from Reference 158... 

In this chapter we discuss the chemistry of germanium, tin and lead species in which at least one organic group is bonded to carbon. Similar silicon compounds will sometimes be discussed for comparison. Synthetic details will be mentioned mostly for the compounds recently reported. Species without any M-carbon bond will also be described. 

On the theoretical side, Schaefer reviewed the early studies on silene that contributed greatly to the theoretical interest in these compounds (35). The review by Raabe and Michl (79) on multiply bonded silicon included a comprehensive examination of the early theoretical work. Gordon (85,86) has summarized multiply bonded silicon, and the studies of Luke et al. deserve mention as a comprehensive study of multiply bonded silicon at a uniform level of theory (87,88). Nagase et al. (89) have reviewed their studies on doubly bonded silicon and germanium, with particular emphasis on reactivity. Most recently, Apeloig (90) has provided encyclopedic coverage of theoretical studies of silicon compounds through the middle of 1987. 

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