The mechanism of the Cu-free allylic alkylation catalysed by NHC-ligand with Grignard reagents was explored with the hybrid B3PW91 DFT method. The bonding nature of the active Mg-species, which was validated on the 13C chemical shift, was found to be highly ionic. In particular a strong nucleophilic Mg-R (R=CH3) bond was explained by the activation of the NHC Lewis base ligand on Mg. The effect of the ethereal solvent was examined using explicit dimethyl ether solvent and Density-based Solvation Model. The adequate representation of solvation is required to properly reproduce the formation of the supposed active species. The mechanism of the allylic alkylation emphasizes the bifunctional role of Mg center by activating the nucleophile and the nucleofuge promoting an SN2-type mechanism. The NHC ligand bonded to Mg is crucial and activates the Mg-R bond favoring the C-C bond formation in the reaction.