The bis(imino)pyridine iron dinitrogen complex ((PDI)-P-IPr)Fe(N-2)(2) catalyzes the hydrogen-mediated reductive cyctization of enynes and diynes with turnover frequencies comparable to those of established precious metal catalysts. Amino, oxygenated, and carbon-based substrates are readily cyclized to the corresponding hetero- and carbocycles with 5 mol % iron and 4 atm H-2 at 23 degrees C. Stoichiometric reactions between selected substrates and the iron compound under a N-2 atmosphere established transfer dehydrogenation from an isopropyl aryl substituent to either the enyne or diyne substrate. In situ monitoring of the catalytic reaction by H-1 NMR spectroscopy coupled with deuterium labeling experiments established rapid cyclization followed by turnover-limiting hydrogenation.