Steric and electronic effects on Si–Ge bond lengths: An experimental and theoretical structural study of Me2Ge(SiCl3)2 and Me3GeSiCl3

The molecular structures of dimethylbis(trichlorosilyl)germane [Me2Ge(SiCl3)2] and trimethyl(trichlorosilyl)germane (Me3GeSiCl3) have been determined in the gas phase. Me3GeSiCl3 was found ab initio to possess C3v symmetry, with a low-lying torsional motion of the SiCl3 group relative to the GeMe3 group. The gas electron diffraction data were modelled with C3 symmetry, although little deviation from C3v symmetry was observed. Me2Ge(SiCl3)2 (C2v symmetry) was also found to have a very low-lying vibrational frequency relating to the rotation of the SiCl3 groups. This led to the gas electron diffraction data being modelled in C2 symmetry, with the observed combined deviation of the SiCl3 groups being 14° from the eclipsed structure calculated ab initio. The Ge–C bond lengths were unaffected by the addition of an extra SiCl3 group to the central germanium atom, but the Ge–Si bond lengths were observed to increase by over 1 pm.