1,1‐Dilithiogermanes 1 a and 1 b were obtained by treatment of the persilyl‐substituted germacyclopropenes with lithium, which cleaved the two Ge−C bonds in the three‐membered ring. The structure of 1 b has been confirmed by X‐ray crystallography and shows a dimeric structure with a rather unusual arrangement of the two Li atoms. The use of 1 in the effective preparation of double‐bonded derivatives of heavier Group 14 elements has also been shown.

After the breaking of the so-called "double-bond rule" 1 by the preparation of the first heavy alkene analogues stable both in the solid state and solution, silene >SidC< 2 and disilene >Sid Si< 3 in 1981, many stable metallenes >EdC< and dimetallenes >EdE< of group 14 elements (E ) heavier group 14 element) have been synthesized to date. 4 However, the chemistry of the stable heteronuclear dimetallenes >EdE′< composed of different group 14 elements heavier than carbon is still not very common, 5,6 although the homonuclear dimetallenes (i.e., disilenes, digermenes, distannenes, diplumbenes) are represented by a number of examples reported by several research groups. 4 Recently, we reported the synthesis of a 1,2-disila-3-germacyclopenta-2,4-diene derivative, which has SidGe and CdC double bonds with a silole structure, and determined the SidGe double bond length for the first time. 7 Very recently, we have also demonstrated the utility of 1,1dilithiosilanes in the preparation of a variety of doubly bonded derivatives of heavier group 14 elements. 8 We now report on a new application of the dilithiosilane derivative, resulting in the successful isolation of the first silastannene with a >SidSn< double bond. Compounds of this type, both stable and transient, were completely unknown until our study. 9 The reaction of bis[di-tert-butyl(methyl)silyl]dilithiosilane 1 8 with dichlorobis(2,4,6-triisopropylphenyl)stannane 10 in dry THF proceeded cleanly to form the corresponding coupling product, 1,1bis[di-tert-butyl(methyl)silyl]-2,2-bis(2,4,6-triisopropylphenyl)-1sila-2-stannaethene 2, as deep violet crystals in 50% isolated yield (Scheme 1). 11

1,1-Dilithiosilane and -germane, R2SiLi2 and R2GeLi2 (R = SiMetBu2) reacted with MesBCl2 to give the unexpected seven-membered ring products, [1,3,2]oxasila- and -germaborepanes, through the intermediate formation of &gt;Si=B– and &gt;Ge=B– doubly bonded species.

The first compounds containing Si=Ga or Si=In double bonds, 1,3-disila-2-gallata- (2-) and -indataallenic anions (3-), were obtained in the form of lithium salts as dark red crystals by the reaction of bis(di-tert-butylmethylsilyl)dilithiosilane 1 with GaCl3 or InCl3. X-ray crystallographic analysis of 2-.[Li(thf)4]+ and 3-.[Li(thf)4]+ showed that the >SiMSi< frameworks (M = Ga, In) are not linear and the two Si=M bond lengths are about 9% shorter than typical Si-M single bonds. In the 29Si NMR spectra, the signals of the terminal Si atoms in 2- and 3- are greatly shifted upfield (2-: -79.9; 3-: -77.6 ppm). The NPA charge distribution for the model compound, [(Me3Si)2SiGaSi(SiMe3)2]- (4-) showed a large part of the negative charge localizing on the two terminal silicon atoms (-0.736 and -0.729). In addition, 2-.[Li(thf)4]+ reacted with MeI to give the corresponding iodogallane 5 nearly quantitatively. This result confirms that the allenic [>SiMSi<]- fragments are highly polarized.

1,1-Bis(di-tert-butylmethylsilyl)-2,2-bis(2,4,6-triisopropylphenyl)-1-germa-2-stannaeth-ene (2), containing a >GeSn< double bond, was prepared as highly air- and moisture-sensitive deep violet crystals by the coupling reaction of bis(di-tert-butylmethylsilyl)dilithiogermane with dichlorobis(2,4,6-triisopropylphenyl)stannane in THF. The germastannene 2 easily undergoes thermal isomerization to form the corresponding symmetrically substituted isomer (E)-1,2-bis(di-tert-butylmethylsilyl)-1,2-bis(2,4,6-triisopropylphenyl)-1-germa-2-stannaethene (3) by the dyotropic 1,2-shifts of the silyl and aryl substituents.