  We have performed a theoretical investigation of the S$^+$($^4$S) +
SiH$_{2}$($^1$A$_1$) reaction, a possible formation route of the HSiS$^+$ and
SiSH$^+$ cations that are alleged to be precursors of interstellar silicon
sulfide, SiS. Electronic structure calculations allowed us to identify the main
reaction pathways for the systems. The reaction has two exothermic channels
leading to the isomeric species $^3$HSiS$^{+}$ and $^3$SiSH$^{+}$ formed in
conjunction with H atoms. The reaction is not characterized by an entrance
barrier and, therefore, it is believed to be fast also under the very low
temperature conditions of interstellar clouds. The two ions are formed in their
first electronically excited state because of the spin multiplicity of the
overall potential energy surface. In addition, following the suggestion that
neutral species are formed by proton transfer of protonated cations to ammonia,
we have derived the potential energy surface for the reactions
$^3$HSiS$^{+}$/$^3$SiSH$^{+}$ + NH$_{3}$($^{1}$A$_1$).
