Structural and Vibrational Properties of Silyl (SiH₃^–) Anions in KSiH₃ and RbSiH₃: New Insight into Si–H Interactions

The alkali metal silyl hydrides <i>A</i>SiH₃ (<i>A</i> = K, Rb) and their deuteride analogues were prepared from the Zintl phases <i>A</i>Si. The crystal structures of <i>A</i>SiH₃ consist of metal cations and pyramidal SiH₃^– ions. At room temperature SiH₃^– moieties are randomly oriented (α modifications). At temperatures below 200 K <i>A</i>SiH₃ exist as ordered low-temperature (β) modifications. Structural and vibrational properties of SiH₃^– in <i>A</i>SiH₃ were characterized by a combination of neutron total scattering experiments, infrared and Raman spectroscopy, as well as density functional theory calculations. In disordered α-<i>A</i>SiH₃ SiH₃^– ions relate closely to freely rotating moieties with <i>C</i>_3<i>v</i> 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 <i>A</i> = K/Rb. In ordered β-<i>A</i>SiH₃ silyl anions are slightly distorted with respect to their ideal <i>C</i>_3<i>v</i> symmetry. Compared to α-<i>A</i>SiH₃ 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-<i>A</i>SiH₃. 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

Hydrogenous Zintl Phase Ba₃Si₄H_<i>x</i> (<i>x</i> = 1–2): Transforming Si₄ “Butterfly” Anions into Tetrahedral Moieties

The hydride Ba₃Si₄H_<i>x</i> (<i>x</i> = 1–2) was prepared by sintering the Zintl phase Ba₃Si₄, which contains Si₄^6– butterfly-shaped polyanions, in a hydrogen atmosphere at pressures of 10–20 bar and temperatures of around 300 °C. Initial structural analysis using powder neutron and X-ray diffraction data suggested that Ba₃Si₄H_<i>x</i> adopts the Ba₃Ge₄C₂ type [space group <i>I</i>4/<i>mcm</i> (No. 140), <i>a</i> ≈ 8.44 Å, <i>c</i> ≈ 11.95 Å, <i>Z</i> = 8] where Ba atoms form a three-dimensional array of corner-condensed octahedra, which are centered by H atoms. Tetrahedron-shaped Si₄ polyanions complete a perovskite-like arrangement. Thus, hydride formation is accompanied by oxidation of the butterfly polyanion, but the model with the composition Ba₃Si₄H is not charge-balanced. First-principles computations revealed an alternative structural scenario for Ba₃Si₄H_<i>x</i>, which is based on filling pyramidal Ba₅ interstices in Ba₃Si₄. The limiting composition is <i>x</i> = 2 [space group <i>P</i>4₂/<i>mmm</i> (No. 136), <i>a</i> ≈ 8.4066 Å, <i>c</i> ≈ 12.9186 Å, <i>Z</i> = 8], and for <i>x</i> > 1, Si atoms also adopt tetrahedron-shaped polyanions. Transmission electron microscopy investigations showed that Ba₃Si₄H_<i>x</i> is heavily disordered in the <i>c</i> direction. Most plausible is to assume that Ba₃Si₄H_<i>x</i> has a variable H content (<i>x</i> = 1–2) and corresponds to a random intergrowth of <i>P</i>- and <i>I</i>-type structure blocks. In either form, Ba₃Si₄H_<i>x</i> is classified as an interstitial hydride. Polyanionic hydrides in which H is covalently attached to Si remain elusive