Compounds of silicon and homologues, 127 Unsaturated compounds of silicon and homologues, 54 Tetrasupersilyl-tristannaallene and -tristannacyclopropene (tBu(3)Si)(4)Sn-3 - Isomers with the shortest Sn=Sn double bonds to date

The dark blue, air- and moisture-sensitive, thermolabile tristannaallene R* Sn-2=Sn=SnR*(2) (5) (R* = SitBu(3)) is prepared by reaction of Sn(OtBu)(2) or Sn[N(SiMe3)(2)](2) with R*Na in pentane/benzene at -25 degrees C. The da rk red-brown, air-sensitive, moisture-insensitive, and thermostable cyclotr istannene R*Sn-4(3) (6) with a -R*Sn=SnR*- moiety as part of a Sn-3 ring is obtained from the reaction of Sn(OtBu)(2) or Sn[N(SiMe3)(2)](2) with R*Na in pentane at 25 degrees C or from the isomerization of 5 at room temperatu re (tau(1/2) = 9.8 h). According to the result of X-ray structural analyses the Sn-3 framework of chiral 5 is bent (156 degrees) and the terminal Sn a toms have pyramidal surroundings. The SnSn double bonds in 5 (2.68 Angstrom ) are shorter than those found for the hitherto structurally investigated d istannenes (2.77-2.91 Angstrom). Even shorter is the double bond in 6 (2.59 Angstrom). The unsaturated Sn atoms here have nearly planar surroundings i n perfect analogy to the carbon atoms in CC double bonds. The SnSn double b ond in 6 can therefore be considered as the first "true" Sn=Sn bond. The st ructures of 5 and 6 can be deduced also from Sn-119- and Si-29-NMR studies in solution.