Communication on silicon and its group homologues. 131. Communication on saturated compounds of silicon and its group homologues. 56. Dimers of the ethenes Me2EMe,EX-CM(SiMe,),?= C(SiMe3)(2) (E = Si, Ge, Sn): How do they form from Me2EX-CM(SiMe3)(2)? How are they structured?

Alkali metal organyls or silyls MR (e.g. LiMe, (LiBu)-Bu-n, Li'Bu, LiPh, Li CH(SiMe3)(2), LiC(SiMe3)(3), NaSi'Bu-3) convert equimolar amounts of bromom ethanes Me2EX-CBr(SiMe3)(2) with E = Si, Ge, Sn and electronegative substit uents X (e.g. F, Br, OPh) in organic solvents (e.g, pentane, diethyl ether, tetrahydrofuran) (i) by a very fast Br/M exchange into the 'metalation pro ducts' Me2EX-CM(SiMe3)(2), which thermolyze under formation of 'cyclobutane s' [-Me2E-C(SiMe3)(2)-](2), and (ii) to a lesser extent by X/R exchange int o 'substitution products' Me2ER-CBr(SiMe3)(2). As shown by trapping experim ents, the unsaturated compounds Me2E=C(SiMe3)(2) play the role of short-liv ed intermediates in both reactions. They are formed from Me2EX-CM(SiMe3)(2) by MX elimination and add the present alkalimetal compounds Me2EX-CM(SiMe3 )(2) = MR' or MR, respectively. The products Me2ER'-CM(SiMe3)(2) with R' = C(EXMe2)(SiMe3)(2), obtained in this way, eliminate MX under formation of t he mentioned 'cyclobutanes'. On the other hand, the compounds Me2ER-CM(SiMe 3)(2) convert unreacted Me2EX-CBr(SiMe3)(2) in Me2EX-CM(SiMe3)(2) under for mation of Me2ER-CBr(SiMe3)(2). Relative rates of both the metalation reacti ons and the salt eliminations are determined. X-ray structure analyses of [ -Me2E-C(SiMe3)(2)-](2) (E = Si, Ge, Sn) prove their 1,3-dielementacyclobuta ne structure with planar four-membered ECEC rings.