Aluminum(I) and magnesium(I) compounds are reported for the C–C σ-bond activation of strained alkylidene cyclopropanes. These reactions result in the formal addition of the C–C σ-bond to 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 α- or β-alkyl migration steps that break the C–C σ-bond. This mechanistic understanding is used to develop the magnesium-catalyzed hydrosilylation of the C–C σ-bonds of alkylidene cyclopropanes.