(p-d) ? Bonding in Fluorosilanes? Gas-Phase Structures of (CH3)4-nSiFn with n = 1-3 and of t-Bu2SiF2

Article abstract of DOI:10.1021/ja00274a006

The gas-phase structures (r_g values) of the methylfluorosilanes (CH3)_4-nSiF_n with n = 1-3 and of di-tert-butyl-difluorosilane, t-Bu2SiF2, have been determined by electron diffraction.In the case of CH3SiF3 the microwave rotational constant was included in the structure analysis.In the methylfluorosilane series a steady decrease of Si-F and Si-C bond lengths is observed with increasing fluorination: Si-F = 1.600(2), 1.586(2), and 1.570(2) Angstroem and Si-C = 1.848(2), 1.836(2), and 1.828(4) Angstroem for (CH3)3SiF, (CH3)2SiF2, and CH3SiF3, respectively.These trends are rationalized by increasing polar contributions and contraction of the silicon valence shell.Ab initio calculations for SiF4 indicate that (p-d) ? bonding is negligible.Substitution of the methyl groups in (CH3)2SiF2 by tert-butyl groups leads to lengthening of Si-F and Si-C bonds and strong variations in the silicon bond angles: Si-F = 1.586(2), 1.606(4) Angstroem; Si-C = 1.836(2), 1.869(3) Angstroem; CSiC = 116.7(6) deg, 125.5(11) deg; and FSiF = 104.6(4) deg, 97.7(8) deg in (CH3)2SiF2 and t-Bu2SiF2, respectively.