The colorless, liq., pyrophoric alkylalkylideneboranes RB:C(SiMe3)2 (I; R = Me, CMe3) are formed from MeOBMeC(SiMe3)3 by elimination of Me3SiOM at 560 Deg and Me3SiF at 490 Deg resp. At ambient temp. I (R = CMe3) can be stored, whereas I (R = Me) cyclodimerized. I (R = CMe3) crystallizes at 225 K; a structure detn. at 120 K reveals the C-B:C skeleton to be linear with BC bond lengths of 154.7 and 136.1 pm. I forms [2 + 2] cycloadducts with 2,6-dimethylpyridine, benzopahenone, and iminoborane Me3CB.tplbond.NCMe3.