Iron[pyridine(diimine)] catalysts contain a redox active ligand in which the central iron atom can coordinate with two simple, unfunctionalized olefin double bindsbonds. The catalyst can be written as a resonance between a structure containing unpaired electrons with the central iron atom in the II oxidation state, and one in which the iron is in the 0 oxidation state. This gives it the flexibility to engage in binding the double bonds as they undergo a cyclization reaction, generating a cyclobutane structure via C-C reductive elimination; alternatively a cyclobutene structure may be produced by beta-hydrogen elimination. Efficiency of the reaction varies substantially depending on the alkenes used, but rational ligand design may permit expansion of the range of reactions that can be catalyzed.<ref>{{cite journal|title=Iron-catalyzed intermolecular [2+2] cycloadditions of unactivated alkenes|author1=Jordan M. Hoyt |author2=Valeria A. Schmidt |author3=Aaron M. Tondreau |author4=Paul J. Chirik |journal=Science|date=2015-08-28|volume=349|issue=6251|pages=960–963|doi=10.1126/science.aac7440}}</ref><ref>{{cite journal|title=As simple as [2+2]|author1=Myles W. Smith |author2=Phil S. Baran |journal=Science|date=2015-08-28|volume=349|issue=6251|pages=925–926|doi=10.1126/science.aac9883}}</ref>
