Bis(1-bromo-2-methylpropenyl)mercury, prepared via Me2CC(Br)Li, was treated with mercuric bromide to give Me2CC(Br)HgBr. The latter transferred Me2CC: to olefins, giving isopropylidenecyclopropanes, in sealed tube reactions carried out at 150°C in the presence of an equimolar quantity of diphenylmercury. The Me2CC(Br)HgBr/Ph2Hg reagent also reacted with triethylsilane at 150°C to give Et3SiCHCMe2