Solution NMR studies of silyl cations [ArSiMe 2] +X − (X = I, CF 3SO 3) incorporating the terdentate aryl diamine ligand Ar - C 6H 3− 2,6-(CH 2NMe 2) 2 have been carried out in a protic solvent (methanol- d 4) and in an aprotic solvent (CD 2Cl 2). This study has shown that the structure of these silyl cations is highly dependent on the solvent. In CD 2Cl 2, the silyl cation is five-coordinated owing to the coordination of one NMe 2 group and of the anion to the silicon centre which gives rise to a dissymmetric structure. On the other hand, in CD 3OD there is no coordination of the anion, but the silyl cation is also probably five-coordinated due to the coordination of the solvent to the silicon atom which is supported by the X-ray analysis of the compound 9. With the weakly nucleophilic anion BPh 4 − in CD 2Cl 2, in addition to the silyl cation previously described, another five-coordinated silyl cation resulting from the coordination of both NMe 2 groups to the Si centre was postulated.