The chloroplatinic acid or platinum(II) bromide-catalyzed reaction of arachno-6,8-C2B7H13 with ethylene results in hydroboration to yield the single product 7-(C2H5)-arachno-6,8-C2B7H12. Analogous reactions with l-pentene or styrene yield a mixture of both hydroboration, 7-R-arachno-6,8-C2B7H12, and dehydrogenative borylation, 7-(trans-R-CH=CH)-arachno-6,8-C2B7H12, products. On the other hand, the palladium(II) bromide-catalyzed reaction of arachno-6,8-C2B7H13 with ethylene yields predominantly the dehydrogenative borylation product (7-(CH2=CH)-arachno-6,8-C2B7H12 along with smaller amounts of 7-(C2H5)-arachno-6,8-C2B7H12. Palladium(II) bromide-catalyzed reactions with l-pentene or styrene result in only dehydrogenative borylation to produce 7-(trans-R-CH=CH)-arachno-6,8-C2B7H12. The hydroboration and dehydrogenative borylation products observed in the platinum- and palladium-catalyzed reactions are related to those that have been observed in recently reported metal-catalyzed hydrosilations and catecholborane or pinacolborane hydroborations and are consistent with a reaction mechanism involving competitive hydride-migration/reductive-elimination and boryl-migration/beta -hydride-elimination steps. Crystallographic and DFT computational studies of the alkenyl-carbaboranes have also revealed unusual cage-bonding features that suggest pi -bonding interactions of the B7 cage boron with its olefinic substituent.