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Zusammenfassung:
The gas-phase molecular structures of NH(SiHMeCl)2 1 and NMe(SiHMeCl)2 2 have been investigated by electron diffraction (ED) and by ab initio geometry optimisations at the MP2/6-31G* level. Ab initio calculations suggest six different conformers to be present for both compounds. They also predict that all conformers exhibit planar or nearly planar configurations at the N atom and that all the Si–Cl bonds are more or less orthogonal to the SiNSi plane, a feature that is supported by the ED refinement. Natural bond orbital analysis and an SCF deletion calculation showed an energy contribution of the n(N)–σ*(Si–Cl) hyperconjugative interaction of more than 80 kJ mol–1, thus rationalising conformations that optimise an overlap of the two appropriate orbitals. The principal parameters (ra) from electron diffraction for NH(SiHMeCl)2 (with estimated standard deviations given in parentheses and ab initio-calculated values for the lowest-energy conformer given in square brackets) are: r(Si–N) 171.2(2) [173.7], r(Si–Cl) 206.2(1) [207.7], r(Si–C) 184.5(3) [186.4] pm; Si–N–Si 128.8(12) [131.3], N–Si–Cl 112.9(4) [111.3], N–Si–C 115.7(9) [112.0] and Cl–Si–C 102.6(6) [107.3]°. For NMe(SiHMeCl)2: r(Si–N) 171.5(1) [173.7], r(Si–Cl) 207.1(1) [208.1], r(Si–C) 185.2(2) [186.8], r(N–C) 147.4(6) [148.1] pm; Si–N–Si 124.9(6) [123.7], N–Si–Cl 108.3(5) [110.4], N–Si–C 115.1(8) [112.4] and Cl–Si–C 107.3(7) [106.7]°.
