The bis(imino)pyridine iron dinitrogen compounds, ((iPr)PDI)Fe(N(2))(2) and [((Me)PDI)Fe(N(2))](2)(mu(2)-N(2)) ((R)PDI = 2,6-(2,6-R(2)-C(6)H(3)N=CMe)(2)C(6)H(3)N; R = (i)Pr, Me), promote the catalytic intermolecular [2 pi + 2 pi] cycloaddition of ethylene and butadiene to form vinylcyclobutane. Stoichiometric experiments resulted in isolation of a catalytically competent iron metallocycle intermediate, which was shown to undergo diene-induced C-C reductive elimination. Deuterium labeling experiments establish competitive cyclometalation of the bis(imino)pyridine aryl substituents during catalytic turnover.