An iron(II) N-heterocyclic carbene (NHC) complex is applied as catalyst for aldehyde olefination with ethyl diazoacetate (EDA) in the presence of triphenylphosphine. The reaction leads to high olefin yields with very good E-selectivities. The key step of the reaction is the catalytic in situ generation of a phosphorus ylide. Mechanistic studies reveal two possible pathways for the formation of the Wittig reagent with respect to the carbene source being the metal carbene (NHC)Fe-IV=CH(CO2Et), and phosphazine, Ph3P=N-N=CH(CO2Et). Based on the experimental observations a new mechanism for the transformation of phosphazine is proposed. (C) 2016 Elsevier Inc. All rights reserved.