C-Pb 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 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... 

In 1958, Panov and Kocheshkov1216 found another route to the formation of the C—Pb bond, namely the interaction of tetraacyloxyplumbanes with aromatic and heteroaromatic compounds (the plumbylation reaction). They showed that the reaction of thiophene with Pb(OCOPr-i)4 at room temperature during 10 days led to unstable RPb(OCOR/)3 (R = 2-thienyl R = i-Pr), which was disproportionated to R2Pb(OCOR/)2 and Pb(OCOR/)4-... 

Among the C—Pb bond cleavage reactions, thermo- and photo-induced homolytic cleavage is of special theoretical and practical interest. 

In 1964, Emeleus and Evans1391 found that the C—Pb bond was the first to be cleaved and the Pb—Pb bond was cleaved next in the reactions of HC1 with R3PbPbR3. The process of formation of Pb( l2 was unclear and hence the reaction mechanism was represented by the two equations 36 and 37. 

Scheme 3 did not require the initial cleavage of the C—Pb bond by A1C13 as well as the intermediate formation of ClR2PbPbR2Cl, which has not yet been identified. 

In contrast, the half-life of R3ST radicals is 10 min at ordinary temperature. It is noteworthy that a photochemical decomposition of [(Me3Si)2CH]2Pb fails to proceed according to equation 26, but leads instead to homolysis of both C—Pb bonds and to the formation of a lead mirror deposition (equation 27). 

This result supports the data presented in Section V.A, that of all C—M bonds, the C—Pb bonds can undergo homolytic cleavage, forming carbon-centred free radicals most easily. 

In the solid state, the adducts are monomeric and the three-coordinate carbene C atom is essentially planar and the Pb atom is pyramidalized with a relatively long carbene C-Pb bond, 2.586(7) A. In solution, the adducts dissociate to free carbenes and plumbylenes. 

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