The first tetrasilabuta‐1,3‐diene (2) was obtained as reddish‐brown crystals simply and unexpectedly by lithiation of disilene 1, partial bromination with an aryl bromide, and intermolecular cleavage of lithium bromide. The conjugation between the two SiSi double bonds of 2 was demonstrated by electron spectroscopy and X‐ray crystallography. Mes = 2,4,6‐Me3C6H2, Tip = 2,4,6‐iPr3C6H2.

The isolation and characterization of thermally stable diaminocarbenes and diaminosilylenes has reawakened interest in the low‐coordinated compounds of group‐14 elements. The existence of these species is presumably due to interactions between the free electron pairs on the nitrogen atoms and the carbene carbon atom or the silicon atom. The present review is mainly concerned with the heavier analogues R2Ge:, R2Sn:, and R2Pb:, systems without intramolecular donor stabilization that owe their existence principally to steric shielding by the voluminous alkyl or aryl groups R. Dimerizations of these electron‐sextet molecules give rise to the double‐bond systems of the digermenes, distannenes, and diplumbenes; the latter species have for a long time been considered as being incapable of existence. The properties of these molecules are compared with the results of quantum chemical calculations. A separate section is devoted to the isomers of Si4R6, which include novel tetrasilacyclobutenes and the diene analogues, the tetrasilabuta‐1,3‐dienes.

The diarylplumbylene R 2 Pb: (3), R ) 2-tBu-4,5,6-Me 3 C 6 H, and the rearranged alkylarylplumbylene RR′Pb:, R ) 2,4,6-tBu 3 C 6 H 2 , R′ ) CH 2 C(CH 3 ) 2 -3,5-tBu 2 C 6 H 3 , were synthesized and characterized by NMR and UV/vis spectroscopy, as well as by X-ray crystallography. Treatment of 3 with the disilylplumbylene R′′ 2 Pb:, R′′ ) Si(SiMe 3 ) 3 , furnished the heteroleptic plumbylene RR′′Pb: (8), which, in the solid state, forms the plumbylene dimer RR′′PbPbRR′′ (9). The X-ray structure analysis of 9 reveals a trans-bent angle of 46.5°and a Pb ... Pb separation of 3.37 Å, the shortest observed so far between the lead atoms of two plumbylenes.

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Tetrakis(2,4,6-triisopropylphenyl)digermene (2), prepared by treatment of germanium bis(trimethylsilyl)amide with 2,4,6-triisopropylphenyllithium, reacts with diazomethane or trimethylsilyldiazomethane in a [2+1] fashion to furnish the N-methyleneamino-substituted azadigermiridines 7 and 8, respectively. The dissociation equilibrium between 2 and bis(triisopropylphenyl)germylene has been confirmed by the trapping reaction with a 1,2-quinone, from which the 1,3-dioxa-2-germaindan 10 was isolated. The structures of 2, 7, 8, and 10 were determined by X-ray crystallography. The sterically congested digermene 2 has a short Ge−Ge double-bond length of 2.213(1) Å.

Bis(2,4,stannylene, formed by thermolysis of the cyclotristannane 4, reacts with 4,5-dimethyl-l,3-diisopropylimidazol-2-ylidene to furnish the adduct 8; an X-ray structural analysis of this compound reveals the presence of a long tin-carbon 'double bond' with a length of 237.9(5) pm and a distinctly pyramidal tin centre.

Although tetrakis(2-tert-butyl-4,5,6-trimethylphenyl)distannene (9), the first tetraaryl-distannene to be reported on, is stable in the solid state, only the monomeric stannylene 8 can b e detected in solution. An X-ray crystal structure analysis of 9 reveals a long tin-tin bond length of 291.0( 1) p m and a trans-bent Sn2C4 framework with different fold angles of 21.4" and 64.4".Lappert's tetraalkyldistannene 1 was the first molecule to be reported on with a homonuclear double bond between heavier elements of the fourth main g r o~p [~,~l . Even though numerous disilene~[~,~] as well as some digermenesL5] -compounds with Si=Si and Ge=Ge double bonds -have been discovered in the meantime, compound 1 has remained as the only structurally characterised distannene. Although 1 is stable in the solid state, it dissociates in solution to two stannylene molecules 2. The distannene 5, accessible from thermal or photochemical reaction of the cyclotristannane 3r61, behaves differently and retains its structural integrity in solution, as evidenced by ' 19Sn-NMR spectroscopy [6] and trapping r e a c t i o n~ [~~~] .However, only the three-membered ring system 2 can be detected in the solid state at room temperature (Scheme l)I6p9I. The apparently promising idea to suppress the formation of the three-membered ring system by replacing the 2,4,6-triisopropylphenyl group in 5 by the even bulkier 2,4,6-tritert-butylphenyl group was not successful since the thus formed bis(2,4,6-tri-tert-butylphenyl)stannylene 6 exists only as the monomer also in the solid state"O1. A weak tintin interaction was observed ["] in one of the crystalline forms of the intramolecular donor atom-stabilised molecule Sn[2,4,6-(CF3)3C6H2]2; however, the separation of 364 pm is much too large for a possible bonding situation['*].Since the steric overcrowding in 6 was obviously too large for distannene formation, we reduced the steric requirements of the substituents and now report on the formation of the stannylene 8 and the distannene 9 which is stable in the solid state. The reaction of the Grignard compound 7 with tin(I1) chloride affords a dark red solution of the stannylene 8, the constitution of which is substantiated by spectral data and trapping reaction with, for example, 3,5-ditert-butylbenzoquinone to yield the [4 + 11 cycloadduct 10. Chem.