The first phosphoniomethylidene complexes of scandium and lutetium, [LLn(CHPPh3)X] (L = [MeC(NDIPP)CHC(NDIPP)Me]-; Ln = Sc, X = Me, I, TfO; Ln = Lu, X = CH2SiMe3), have been synthesized and fully characterized. DFT calculations clearly demonstrate the presence of an allylic Ln, C, P π-type interaction in these complexes. X-ray diffraction indicates that the scandium iodide complex has the shortest Sc-C bond length to date (2.044(5) Å). These phosphoniomethylidene complexes readily convert into the ylide complexes, and the reactivity is affected by both X- anion and Ln3+ ion. The reaction of lutetium complex with imine shows a rapid insertion of imine into the Lu-C(alkylidene) bond. DFT calculations indicate that, although the bonding situation seems similar to that of the scandium analog, the strong negative charge at the alkylidene carbon is not sufficiently screened by one hydrogen in the lutetium complex because of a more ionic bonding, and therefore, the reactivity of the lutetium complex is much higher.