The catalytic transformation of a C(sp(3))-H bond to a C(sp(3)) C bond via an iron carbene intermediate represents a long-standing challenge. Despite the success of enzymatic and small molecule iron catalysts mediating challenging C(sp(3))-H oxidatiiins and aminations via high-valent iron oxos and nitrenes, C(sp(3))-H alkylations via isoelectronic iron carbene intermediates have thus far been unsuccessful. Iron carbenes have been inert, or shown to favor olefin cyclopropanation and heteroatom-hydrogen insertion. Herein we, report an iron phthalocyanine-catalyzed alkylation of allylic and benzylic C(sp(3))-H bonds. Mechanistic investigations support that an electrophilic iron carbene mediates homolytic C-H cleavage and rebounds from the resulting organoiron intermediate to form the C C bond; both steps are tunable via catalyst modifications. These studies suggest that for iron carbenes, distinct from other late metal carbenes, C-H cleavage is partially rate-determining and must be promoted to effect reactivity.