Abstract1,1‐Organoboration, using triethyl‐, triallyl‐ and triphenyl‐borane (BEt3, BAll3, BPh3), of dimethysilylethynyl(trimethyl)stannane, Me3SnCCSi(H)Me2 (1), affords alkenes bearing three different organometallic groups at the CC bond. For BEt3 and BPh3, the first products are the alkenes 4 with boryl and stannyl groups in cis‐positions. These rearrange by consecutive 1,1‐deorganoboration and 1,1‐organoboration into the isomers 5 as the final products, where boryl and silyl groups are in cis‐positions linked by an electron‐deficient SiHB bridge. 1,1‐Ethylboration of bis(dimethylsilylethynyl)dimethylstannane, Me2Sn[CCSi(H)Me2]2 (2), leads to the stannacyclopentadiene 6 along with non‐cyclic di(alkenyl)tin compounds 7 and 8. 1,1‐Ethylboration of ethynyl(trimethylstannylethynyl)methylsilane, Me(H)Si(CCSnMe3)CCH (3), leads selectively to a new silacyclopentadiene 13 as the final product. The reactions were monitored and the products were characterized by multinuclear magnetic resonance spectroscopy (1H, 11B, 13C, 29Si and 119Sn NMR). Copyright © 2005 John Wiley & Sons, Ltd.