(F3C)F2SiONMe2 was prepared from LiONMe2 and F3CSiF3. It was characterized by gas IR and multinuclear solution NMR spectroscopy and by mass spectrometry. Its structure was elucidated by single crystal X-ray crystallography and by gas electron diffraction. (It exists as a conformer mixture.) Important findings were extremely acute SiON angles [solid 74.1(1) degrees , gas anti 84.4(32) degrees and gauche 87.8(20) degrees] and short Si...N distances [solid 1.904(2) A]. The bending potential of the SiON unit was calculated at the MP2/6-311++G(3df,2dp) level of theory and appears very flat and highly asymmetric. The calculated atomic charges (NPA) are counterintuitive to the expected behavior for a classical Si-N dative bond, as upon formation of the Si...N bond electron density is transferred mainly from oxygen to nitrogen, while the silicon charge is almost unaffected. Despite the molecular topology of a three-membered ring, the topology of the electron density shows neither a bond critical point between Si and N atoms nor a ring critical point, but the electron density and Laplacian values are related to other hypercoordinate Si compounds. The electronic properties of (F3C)F2SiONMe2 were compared to those of the adduct (F3C)F2(MeO)Si-NMe3, whose properties and structure were also calculated. The charge distribution and Laplacian values along the Si-N vectors in both molecules are similar but not equivalent. (F3C)F2SiONMe2 contains thus a nonclassical Si...N bond, and its properties can be regarded as a new model for the explanation of the old postulate of an alpha-effect in silicon chemistry, explaining the behavior of compounds with geminal Si and N atoms.

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.

The simple silylhydrazines F(3)SiN(Me)NMe(2) (1), F(2)Si(N(Me)NMe(2))(2) (2), and F(3)SiN(SiMe(3))NMe(2) (3) have been prepared by reaction of SiF(4) with LiN(Me)NMe(2) and LiN(SiMe(3))NMe(2), while F(3)SiN(SnMe(3))NMe(2) (4) was prepared from SiF(4) and (Me(3)Sn)(2)NNMe(2) (5). The compounds were characterized by gas-phase IR and multinuclear NMR spectroscopy ((1)H, (13)C, (14/15)N, (19)F, (29)Si, (119)Sn), as well as by mass spectrometry. The crystal structures of compounds 1-5 were determined by X-ray crystallography. The structures of free molecules 1 and 3 were determined by gas-phase electron diffraction. The structures of 1, 2, and 4 were also determined by ab initio calculations at the MP2/6-311+G** level of theory. These structural studies constitute the first experimental proof for the presence of strong Si.N beta-donor-acceptor bonds between the SiF(3) and geminal NMe(2) groups in silylhydrazines. The strength of these non-classical Si.N interactions is strongly dependent on the nature of the substituent at the alpha-nitrogen atom of the SiNN unit, and has the order 3>4>1. The valence angles at these extremely deformed alpha-nitrogen atoms, and the Si.N distances are (crystal/gas): 1 104.2(1)/106.5(4) degrees, 2.438(1)/2.510(6) A; 3 83.6(1)/84.9(4) degrees, 2.102(1)/2.135(9) A; 4 89.6(1) degrees, 2.204(2) A.

The crystal structures of (H(3)C)(2)O, H(2)SiCl(2) and an adduct of these were determined by low-temperature X-ray crystallography on crystals grown in situ at low temperatures on a diffractometer. The adduct of (H(3)C)(2)O and H(2)SiCl(2) has the composition [(H(3)C)(2)O.H(2)SiCl(2)](2) and contains a four-membered Si(2)O(2) ring, with the Cl atoms pointing to the outside and the Si-H functions pointing to the inner side of the ring. The Si(2)O(2) ring has two longer and two shorter SiO bonds and thus deviates from a square. Quantum chemical calculations give a geometry for [(H(3)C)(2)O.H(2)SiCl(2)](2) which has D(2h) symmetry and allow to obtain an estimate for the adduct formation energies, which are -13.4 kJ mol(-1) for the formation of the mono adduct [(H(3)C)(2)O + H(2)SiCl(2)-->(H(3)C)(2)O.H(2)SiCl(2)], -14.4 kJ mol(-1) for the dimerization of two mono adducts [(H(3)C)(2)O.H(2)SiCl(2)-->[(H(3)C)(2)O.H(2)SiCl(2)](2)] and -41.2 kJ mol(-1) for the reaction 2 (H(3)C)(2)O + 2 H(2)SiCl(2)-->[(H(3)C)(2)O.H(2)SiCl(2)](2). The results are used to rationalize the strongly reduced reactivity of H(2)SiCl(2) towards nucleophilic substitution reactions in (H(3)C)(2)O at low temperatures.