Silicon, Germanium and Tin Compounds

A number of organosilicon compounds with chiral silicon atoms have been obtained as single enantiomers by routes involving  

Examples are shown in 24-26. The stereospecificity of substitution reactions at silicon indicates a viable route to other enantiomerically pure silicon compounds. All examples of chiral molecules of the type R R2R3R4Si and R1R2R3SiH have high configurational stability, and in addition halides such as PhSi(Me)(Et)Br do not readily racemize (see Corriu et aid). 

Single enantiomers of a small number of organogermanium compounds, with a chiral germanium atom, have been isolated and an example is the configurationally stable compound 27 (see Marshall and Jablonski5). 

Tetraorganotin compounds with a stereogenic tin atom have been obtained as single enantiomers variously by resolution, asymmetric synthesis or, as in the example of 28, separation of the enantiomers has been achieved by chiral chromatography on cellulose acetate. Compounds such as 28 exhibit no tendency to racemize, in contrast to 

6 Amines, Ammonium Salts, Phosphorus and Arsenic Compounds 

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Rzaev, Z. M. et al. Third All-Union Conf. on Biologically Active Compositions of Silicon, Germanium and Tin Compounds, Abstr., p. 63, Irkutsk 1980... 

The single-bond compounds between Si, Ge, and Sn and chalcogens (S, Se, Te) are reactive and used as synthetic intermediates in the synthesis of organic compounds, organochalcogen compounds, organo-silicon, -germanium and -tin compounds, and transition metal-chalcogen complexes and clusters. 

In later work26 the effect of the metal atom was investigated and rate coefficients (k2) calculated for silicon, germanium, and tin compounds. These are given in Table 10. 

Consequently, the ionization process is a specific phenomenon caused by the lability of the Si-Br or Si-I bonds and the affinity of silicon for oxygen not all racemization processes involve ionic 1 1 adducts. Moreover, the case of the tin compounds shows that the ability to undergo racemization is connected with the ability to increase coordination number. Thus, since silicon, germanium, and tin compounds show similar conductimetric and kinetic behavior, we may conclude that racemization and nucleophilic substitution are both activated by nucleophiles and involve a coordination extension process as shown in Scheme 67. However, existence of the siliconium ion 209 cannot be excluded in the case of bromosilanes. 

Petrov and Mironov267 have reviewed recent investigations of organo-silicon, -germanium, and -tin compounds, especially those containing unsaturated hydrocarbon groups such as vinyl and ethynyl, including their preparation and reactions. 

One relevant review has appeared Dynamic stereochemistry of hypervalent silicon, germanium and tin compounds containing amidomethyl C,0-chelating ligands . 

C. L. De Ligny and N. G. Van der Veen, Rec. Trav. Chim., 90, 984 (1971). Solubilities of some tetra-alkyl carbon, -silicon, -germanium, and -tin compounds in mixtures of water with methanol, ethanol, dioxane, acetone and acetic acid and differences between the standard chemical potentials of these solutes in their solutions in water and in the mixed or nonaqueous solvents at 25°C. 

The oxidation state -1-4 is predominantly covalent and the stability of compounds with this oxidation state generally decreases with increasing atomic size (Figure 8.1). It is the most stable oxidation state for silicon, germanium and tin, but for lead the oxidation state +4 is found to be less stable than oxidation state +2 and hence lead(IV) compounds have oxidising properties (for example, see p. 194). 

Sekiguchi, Akira, and Sakurai, Hideki, Cage and Cluster Compounds of Silicon, Germanium, and Tin. 37 1... 

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. 

For quadricovalent silicon, germanium, and tin (and also for atoms such as nitrogen in a substituted ammonium ion) the same tetrahedral orientation of bonds is expected, since 3a-3p, 4a-4p, and 5 -5p hybridization is the same as that for the 2s-2p system. Observed values of bond angles in unsymmetrical compounds of these substances are also included in Table 4-3. 

A bond between two lead atoms is found46 in hexamethyl dilead, Pbj(CH ) . The Pb—Pb bond length, 2.88 0.03 A, is about that expected from the tetrahedral radius of lead, as is the Pb—C bond length, 2.25 0.0G A. This substance is, of course, similar to hexa-methyletham and the corresponding compounds of silicon, germanium, and tin. 

Also given in Table 1 are the strain energies of the still heavier germanium and tin compounds calculated at the HF/DZ(d) level14. Substitution of carbon or silicon by germanium... 

E. A. Chernyshev, M. V. Reshetova and A. D. Volynskikh, Silicon-, Germanium- and Tin-containing Derivatives of the Transition Elements Compounds, NIITEKhIM, Moscow, 1975. 

A. N. Egorochkin and M. G. Voronkov, Electronic Structure of Organic Compounds of Silicon, Germanium and Tin, Publishing House of the Siberian Branch of Russian Academy of Sciences, Novosibirsk, 2000 (Russian). 

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