The LUMO of [CF3P−W(CO)5] is lower in energy than the LUMO of [ClP−W(CO)5] by 0.30 eV and [CF3P−W(CO)5] is the most electrophilic phosphinidene tungstenpentacarbonyl complex described so far. It is generated by reaction of dimethyl acetylenedicarboxylate with the appropriate 1-trifluoromethylphosphole complex at 80 oC in toluene. It reacts normally with alkynes to give 1-trifluoromethylphosphirene complexes. The reaction with alkenes works only with electron-rich species. The phosphirane complexes appear to be unstable when electron-poor C=C double bonds are used such as with trans-stilbene. This highlights the importance of the P to C=C backbonding in the stability of the phosphirane adducts. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim