Plumbylenes structure

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... 

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

The progress in the chemistry of plumbylenes is far behind those of its lighter congeners (silylenes, germylenes, and stannylenes). Most of the recent reports on plumbylenes are describe their synthesis and structural analysis there have been very few descriptions of their reactivities. 

Direct synthesis of complexes of structure type B from germylenes, stannylenes and plumbylenes is possible in several cases. Many complexes of the very sterically hindered metallenes M[CH-(SiMe3)2]2 (M = Ge, Sn, Pb) have been obtained (Scheme 1).2 27... 

Finally, we shall briefly discuss the compounds of the silicon subgroup elements in their divalent state (germylenes, stannylenes and plumbylenes) R2M (R = alkyl, aryl). The chemical bonds in R2M are formed by the px and py orbitals of M. The pz orbital is unoccupied and there is an unshared electron pair in an s-orbital of M. Therefore, the R2M compounds have both electrophilic and nucleophilic properties. The valence angle R—M—R diminishes as the atomic number of M increases. The distances of M—R bonds in R2M are less than in the corresponding tetracoordinate R4M derivatives87. The spatial structure of R2M compounds, the shielding of the reaction centre (M ) as well as the possibility for R2M molecules to transit from a singlet state into a triplet state depend on the steric bulk of the R substituents. If the R substituents are bulky, some R2M ... 

The first bis(organyl)plumbylene isolated and structurally characterized (at first in the gas phase only) was the dark blue bis[bis(trimethylsilyl)methyl]plumbylene Bsi2Pb 3046. Recently we were able to determine the crystal structure of the low-melting solid and, to our surprise, found (very) weakly associated dimers (Figure 6)49. Despite a Pb Pb distance not much shorter than twice the van der Waals radius of Pb, the dimer adopts a trans-bent conformation typical for the heavier congeners of the alkenes. In spite of the same conformation (syn, syn) in the gas phase and in the solid state, the structure displays markedly differing C—Pb—C angles of 103.6° (30) and 93.6°((30)2), respectively. 

FIGURE 6. Molecular structure of plumbylene 30 in the crystal formation of loose dimers... 

FIGURE 7. Molecular structure of plumbylene 8 in the crystal short FLC- -Pb contacts are depicted as wide (=) sticks... 

FIGURE 8. Molecular structure of the heteroleptic plumbylene 43a in the crystal... 

FIGURE 14. Plot of the central part of the molecular structure of trimeric chloro(aikyl)plumbylene (63)3 in the crystal, showing its unusual boat conformation... 

A stable zwitterionic carbene-plumbylene adduct has been reported (equation 64)80. According to its X-ray crystal structure, the central carbon atom is almost planar while the lead atom is pyramidal. As expected, the P — C+ bond length (2.54 A) is longer than a covalent P—C (about 2.38 A) and a P=C bond, which is expected to be about 2.05 A80. 

Besides the triatomic CAs, the molecular structures have been determined experimentally for only labile silylidene and germylidene, as shown in Table 10. At the same time most of the stable CAs have been characterized by X-ray analysis or by electron diffraction. The available structural data for some stable germylenes, stannylenes and plumbylenes have partly been presented in Section IV. The comprehensive consideration of the geometries of stable CAs is beyond the scope of the present review. 

Experimental aspects of the dimerization of germylenes, stannylenes and plumbylenes were discussed in a review363. Relations between the characteristics of ER2 (E = C, Si, Ge, Sn, Pb R = H, F) and the structure of their dimers E2R4 were studied theoretically by Trinquier and coworkers364-367. 

This review shows that despite the huge amount of information accumulated on fundamental structural, electronic and spectral characteristics of germylenes, stannylenes and plumbylenes, many gaps still exist in this area. Modem quantum-chemical calculations partially help to fill these gaps, but reliable experimental data are needed. 

In order to elucidate the relationship between the structure of plumbylene dimers and the bulkiness of substituents, Weidenbruch and coworkers synthesized and characterized... 

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