Metallation of two analogous N- and P-allyl molecules Ph₂NCH₂CHCH₂1 and Ph₂PCH₂CHCH₂2 with nBuLi have shown contrasting reactivities based on the choice of Lewis donor. With 1 metallation of the alpha carbon atom was achieved regardless of the Lewis donor used while in comparison metallation of 2 showed an unexpected donor denticity dependence with P–C bond clevage induced with the tri-dentate PMDETA. Complementary DFT and solution studies rationalise this outcome.