Stable plumbylenes

In 1974 the first stable diaminoplumbylene (76) was synthesized by Lappert and coworkers68 and since then many other stable plumbylenes with heteroatom substituents have been reported. Recently, the synthesis and characterization of a stable aryl(arylthio)-plumbylene (77), which is one of the rare examples of heteroleptic plumbylenes, have also been reported (Scheme 29)69. 

Stable plumbylenes that are not strictly organo derivatives are more well known. The first of these to be prepared and fully characterized was Cp2Pb . Others, such as Pb[N(SiMe3)2]2 , Pb(PR2)2 (where R = bulky silyl ligand) , and (tmtaa) Pb(tmtaa = dibenzotetramethyltetraaza[14]annulene] are also known. Their reactions are characterized by facile ligand displacement to form inorganic plumbylenes. 

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... 

Although dicyclopentadienyllead(II) compounds, formally called plumbylenes, have been known since 1956, " they are not the congeners of carbenes since they are stabilized by r] -coordination of cyclopentadienyl ligands. In 1974, the first stable diaminoplumbylene [(Me3Si)2N]2Pb (173) was synthesized by Lappert and... 

Note also the reactivity of the heteroleptic plumbylene 174 toward carbon disulfide. Treatment of 174 with excess amount of carbon disulfide resulted in a formation of unexpected product, lead(II) bis(aryl trithiocarbonate) (192), as a yellow, air- and moisture-stable solid (Scheme 14.85). The reaction must involve not only the insertion of carbon disulfide into the Pb—S bond, but also the formal insertion of a sulfur atom into the Pb C(Tbt) bond and subsequent insertion of another... 

Since plumbylene is a stable state in lead chemistry, oxidative addition of organo-metallics to plumbylene is an attractive way to lead-centered anions. Following this route, multidecker anions can be prepared for lead, if crown or cryptand ligands coordinate the alkali metal cations (equation 62)70. 

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. 

This section is devoted to the electronic spectra of germylenes, stannylenes and plumby-lenes carrying polyatomic substituents. The available data on the absorption maxima of these species are collected in Table 4, from which it can be seen that only stable polyatomic stannylenes and plumbylenes have been characterized by UV spectroscopy. 

UV spectra of stable germylenes, stannylenes and plumbylenes were recorded in solutions or in the solid phase under inert atmosphere in order to prevent access of oxygen and moisture. Thermochromic transitions have been revealed for many stable solid germylenes, stannylenes and plumbylenes. Their nature has not been studied. 

Similarly to the case of the triatomic species (Section III), comparison of absorption maxima of stable germylenes, stannylenes and plumbylenes bearing the same substituents at different divalent atoms of the Group 14 elements (Table 4) does not reveal any... 

The number of polyatomic germylenes, stannylenes and plumbylenes characterized by their vibrational spectra is still very limited. Only IR spectroscopy was used for this purpose. Unstable molecules were studied in low-temperature inert matrices. The stable germylenes, stannylenes and plumbylenes were treated by standard means. 

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. 

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