Treatment of ((PDI)-P-iPr)Fe(N-2)(2) ((PDI)-P-iPr, 2,6-(2,6-iPr(2)C(6)H(3)N=CMe)(2)C5H3N) with CO2 and ethylene resulted in the formation of a homologous series of saturated and unsaturated iron carboxylate products, ((PDI)-P-iPr)Fe(O2CR), the distribution of which depends on the ratio of the reagents. The solid-state and electronic structures of a saturated product, ((PDI)-P-iPr)Fe(O2CC2H5), were elucidated. Product distributions, deuterium labeling studies, and stoichiometric experiments support initial formation of a five-membered metallalactone intermediate, which undergoes subsequent ethylene insertions to generate macrocyclic metallalactones. Competitive beta-hydride elimination, CO2 insertion, or reaction with H-2 determines the fate of the metallalactone, the latter accounting for formation of iron complexes with saturated carboxylates. Similar reactivity was observed upon addition of propiolactone and ethylene to ((PDI)-P-iPr)Fe-(N-2)(2), supporting C-O oxidative addition and C-C bond formation through metallacycle intermediates.