C-Ge bond

The C—Ge bond is less stable toward heterolytic and homolytic cleavage reactions than the C—Si bond, but it is more stable than the C—Sn and C—Pb bonds. This is consistent with the bond energies of these bonds (see Chapter 2). 

In the early 1950s Anderson315,317 used bromine, or bromine and iodine halides in the presence of iron powder (i.e. FeX3 formed in situ) to cleave the C—Ge bond. 

It is noteworthy that under the action of sulfuric acid the C—Ge bond of (PhCH2)4Ge was not cleaved, and (HS03C6H4CH2)4Ge was formed145. In the early 1960s it was shown that the C—Ge bond could be cleaved by AICI3323 and particularly easily by GaCl3 and InCl3. 

In 1963 Razuvaev, Vyazankin and coworkers324 325 found that alkyl halides in the presence of AICI3 cleaved the C—Ge bond in tetraalkylgermanes to give trialkylhalogermanes in good yield. This reaction was later used by other investigators326,327. 

In 1968, Mazerolles and coworkers451 found that selenium inserted into the C—Ge bond of octaorganylgermacyclobutanes R2Ge(CR2)3 gave octaorganyl-l-seleno-2-germcyclo-pentane. 

Hypervalent germanium derivatives are compounds having penta-, hexa- and sometimes heptacoordinate germanium atom. Numerous publications are devoted to inorganic and pseudo-organic (with no C—Ge bonds) derivatives of this type159,488,518,524-528. 

In the first half of the 20th century it was shown that the C—Sn bond in organotin compounds, especially in tetraorganylstannanes, was easily cleaved by both heterolytic and homolytic mechanisms. This fact makes the C—Sn bond quite different (regarding its thermal and chemical stability) from the C—Si and C—Ge bonds and brought it close to the C—Pb bond. In 1945, Waring and Horton835 studied the kinetics of the thermal decomposition of tetramethylstannane at 440-493 °C, or at 185 °C at a low pressure... 

The rotation barrier around the C=Ge bond is less than 5 kcalmol-1335 mAt HF/DZ+P (ECP for Ge) from Reference 336. 

For a given class of compounds, members with C—Si and C—Ge bonds have higher thermal stability and lower reactivity than those with bonds to Sn and Pb. In catenated compounds similarly, Si—Si and Ge—Ge bonds are more stable and less reactive than Sn—Sn and Pb—Pb bonds for example, Si2Me6 is very stable, but Pb2Me6 blackens in air and decomposes rapidly in CCI4, although it is fairly stable in benzene. 

The first example of heterolytic cleavage of the C—Ge bond was the cleavage of tetraorganylgermanes (and later of organylhalogermanes) by halogens or hydrogen halides (mainly Br2 and HBr). A synthetic method of organylhalogermanes (R4 GeX ,w = 1-3) based on this reaction has been widely used. It was first used in 1927 in the laboratories of Kraus and Dennis " and afterwords by many chemists ". ... 

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