Plumbylene

It has become common to classify all molecular compounds, which fulfill the above characteristics, as carbene analogs 9,13>. As a consequence, compounds of divalent silicon, germanium, tin, and lead may be regarded as carbene-like and are therefore called silylenes, germylenes, stannylenes, and plumbylenes. In contrast to carbenes they have one property in common the energetically most favorable electronic state is the singlet 1a2 found by experiments and calculations 9). 

X-ray crystallographic analysis reveals that plumbylene 141 exists as a monomer, and there is no intermolecular interaction between the lead and sulfur atoms,643 while other heteroatom-substituted plumbylenes so far known exist as a heteroatom-bridged cyclic oligomer.65... 

This interpretation of the experimental results was corroborated by ab initio calculations on the relative stabilities of a series of double bond compounds, [H2Pb=X] and their plumbylene type isomers, [frans-H-Pb-X-H] and [ciT-H-Pb-X-H] (X = O, S, Se, and Te) (vide supra) 6 The results of the theoretical calculations and experiments for lead-chalcogen double bond species are in sharp contrast to those of the other Group 14 element analogues which do not isomerize to the divalent compounds.14b... 

Reduction of the acetate (CH3)3SiCH2 2Pb(OAc)2 yields a reactive plumbylene in the first step which reacts further as shown below ... 

The last two reactions are faster than the disproportionation of RHg and the solvolysis of plumbylene. R3PbHgR reacts further by three routes ... 

In the solid state, both 46 and 47 exist as bent monomeric sandwich complexes with ring centroid-lead-ring centroid angles of 152.2° in 46 and 144.2° in 47. Interestingly, if the reaction between PbCl2 and [Li(dme)2][/Bu2G2P3] is carried out in tmen rather than DME, the product formed is not a homoleptic plumbylene but rather the novel heterobimetallic compound 48.73... 

Although Lappert et al. reported on the isolation of the purple dialkylplumbylene 65 as early as 1973, its structure has only recently been determined. In the solid state, compound 65 exists as a loosely bonded dimer with a second plumbylene molecule and has a Pb- -Pb separation of 412.9 pm, with trans-bent angles between the PbC2 plane and the Pb- -Pb vector of 34.2°.82... 

In contrast, the structurally similar molecules 66 and 67 are strictly monomeric in the solid state, with large distances to the lead atoms of neighboring plumbylene molecules. The same holds for the diarylplumbylene 6885 and for the sterically extremely overcrowded plumbylenes 69,86 70,87 as well as 71.88... 

Pb- -Pb interaction is formed. The reaction of the more crowded aryllead bromide [Pb(/r-Br)R]2, R = 2,6-(2,4,6-/Pr3C6H2)C6H3, with 4-isopropylbenzylmagnesium bromide or LiSi(SiMe3)3, yielded strictly monomeric plumbylenes.91... 

The reaction of lead(n) chloride with 2,4,6-triisopropylphenylmagnesium bromide furnished red crystals of compound 84, the first molecule with a short Pb=Pb double bond length of 305.15(3) pm and trans-bent angles of 43.9° and 51.2°.96,96a Compound 84 is stable in the solid state. In solution, it dissociates into the plumbylene molecules 85 that, for example, react with a nucleophilic carbene to furnish the highly labile, zwitterionic adduct 86 (Scheme 2).97... 

However, reaction of the sterically congested arylcopper compound 89 with the disilylplumbylene 66 yielded the strictly monomeric plumbylene 90 (Equation (30)).100... 

Figure 3 (a) Structure of 96. (b) Interaction of three singlet plumbylenes. 

Cyclic plumbylenes, 5,5-dimethyl-l-trimethylsilyl-3-R-l,3,2P-diazaplumbolidines (R = Me3Si,... 

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