The alkali metal silyl hydrides ASiH3 (A = K, Rb) and their deuteride analogues were prepared from the Zintl phases ASi. The crystal structures of ASiH3 consist of metal cations and pyramidal SiH3- ions. At room temperature SiH3- moieties are randomly oriented (α modifications). At temperatures below 200 K ASiH3 exist as ordered low-temperature (β) modifications. Structural and vibrational properties of SiH3- in ASiH3 were characterized by a combination of neutron total scattering experiments, infrared and Raman spectroscopy, as well as density functional theory calculations. In disordered α-ASiH3 SiH3- ions relate closely to freely rotating moieties with C3v symmetry (Si-H bond length = 1.52 Å; H-Si-H angle 92.2 °). Observed stretches and bends are at 1909/1903 cm-1 (ν1, A1), 1883/1872 cm-1 (ν3, E), 988/986 cm-1 (ν4, E), and 897/894 cm-1 (ν2, A1) for A = K/Rb. In ordered β-ASiH3 silyl anions are slightly distorted with respect to their ideal C3v symmetry. Compared to α-ASiH3 the molar volume is by about 15% smaller and the Si-H stretching force constant is reduced by 4%. These peculiarities are attributed to reorientational dynamics of SiH3- anions in α-ASiH3. Si-H stretching force constants for SiH3- moieties in various environments fall in a range from 1.9 to 2.05 N cm-1. These values are considerably smaller compared to silane, SiH4 (2.77 N cm-1). The reason for the drastic reduction of bond strength in SiH3- remains to be explored.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry