(Dimethylaminomethyl)trifluorosilane, Me2NCH2SiF3--a model for the alpha-effect in aminomethylsilanes

F3SiCH2NMe2 was prepared as a model for the investigation of the nature of the alpha-effect in alpha-aminosilanes, by fluorination of Cl3SiCH2NMe2 with SbF3. Under less mild conditions Si--C bond cleavage was also observed, leading to the double adduct F4Si(Me2NCH2SiF3)2, which was characterised by a crystal structure analysis showing that the central SiF4 unit is connected to Me2NCH2SiF3 via SiN dative bonds and FSi contacts. F3SiCH2NMe2 was characterised by multinuclear NMR spectroscopy (1H, 13C, 15N, 19F and 29Si), gas-phase IR spectroscopy and mass spectrometry. It is a dimer in the crystal (X-ray diffraction, crystal grown in situ), held together by two Si--N dative bonds. In solution and in the gas phase the compound is monomeric. The structure of the free molecule, determined by gas-phase electron diffraction, showed that, in contrast to former postulates, there are no attractive SiN interactions. Ab initio calculations have been carried out to explain the nature of the bonding. F3SiCH2NMe2 has an extremely flat bending potential for the Si-C-N angle; the high degree of charge transfer from the Si to the N atoms which occurs upon closing the Si-C-N angle is in the opposite direction to that expected for a dative bond. The topology of the electron density of F3SiCH2NMe2 was analysed. Solvent simulation calculations have shown virtually no structural dependence on the medium surrounding the molecule. The earlier postulate of Si-->N dative bonds in SiCN systems is discussed critically in light of the new results.