  Hydrogen is rarely observed as ligand in hypervalent species, however, we
find that high-pressure hydrogenation may stabilise hypervalent hydrogen-rich
materials. Focussing on ternary silicon hydrides via lithium doping, we find
anions composed of hypervalent silicon with H ligands formed under high
pressure. Our results reveal two new hypervalent anions: layered-SiH$_{5}^{-}$
and tricapped trigonal prismatic SiH$_{6}^{2-}$. These differ from octahedral
SiH$_{6}^{2-}$ described in earlier studies. In addition, there are further
hydrogen-rich structures Li$_{3}$SiH$_{10}$ and Li$_{2}$SiH$_{6+\delta}$ which
may be stabilised at high pressure. Our work provides pointers to future
investigations on hydrogen-rich materials.
