Aluminum(I) and magnesium(I) compounds are reported for the C-C sigma-bond activation of strained alkylidene cyclopropanes. These reactions result in the formal addition of the C-C sigma bond to the main group center either at a single site (Al) or across a metal-metal bond (Mg-Mg). Mechanistic studies suggest that rather than occurring by a concerted oxidative addition, these reactions involve stepwise processes in which substrate binding to the main group metal acts as a precursor to alpha- or beta-alkyl migration steps that break the C-C sigma bond. This mechanistic understanding is used to develop the magnesium-catalyzed hydrosilylation of the C-C sigma bonds of alkylidene cyclopropanes.