The exhaustive hydroborations of the (C.tplbond.C)-groups in Me2Si(C.tplbond.CMe)2 (A) by adding ethyldiborane(6) at room temp. is presumed to lead initially to the formation of a mixt. of the threo- and erythro-3,3,5,6-tetrakis(diethylboryl)-4,4-dimethyl-4-silaheptanes (1a, b). The threo-1a reacts further by BH borane catalyzed intermol. condensation to the substituted disilatetraboratricyclo[126.96.36.199,9]dodecane I, whose crystal structure [space group C2/c, a = 19.696(2), b = 10.371(1), c = 16.580(2) .ANG.; b = 125.90(1) Deg; at 122 K] has been established by x-ray diffraction. In contrast, the erythro-1b undergoes intramol., thermal elimination of Et3B to give 1,2-diethyl-2,4-bis(diethylboryl)-3,3,5-trimethyl-3-silaborolane II. If A is added to excess of undiluted (Et2BH)2, then two substituted diastereomers of 1-carba-arachno-pentaboranes(10), are formed preferentially as the result of an initial Si-C.tplbond.-cleaved hydroboration.