The iron Lewis acid, [(;75-C5H5)Fe(CO)2(THF)]+[BF4]-) was found to be an effective catalyst for the preparation of aziridines. This new method provides a facile, one-step route to predominantly cisaziridines, with yields up to 95%, from compounds with a diazo functionality and a variety of substituted AT-benzylidene imines with N-aryl or AT-alkyl groups. The reaction mechanism is believed to proceed through an electrophilic iminium ion intermediate. To support this idea, the iron Lewis acid-imine complex [(?75-C5H5)Fe(CO)2(PhCH=NPh)]+[BF.j]- was prepared, characterized, and reacted with different diazo compounds to provide the resultant czs-aziridines. Alternatively, it may be possible that the aziridines were derived from an electrophilic carbenoid intermediate, as is often proposed. Thus, the iron carbene [(2-CsIWFeCCOMCHPhOl+LSOaCFg]- was prepared and treated with AT-benzylideneaniline; however, the resultant aziridine was not formed.