Answer
NiS is more soluble in ammonia because the formation of the [Ni(NH$_{3}$)$_{6}$]$^{2+}$ complex ion is highly favorable. The formation removes Ni$^{2+}$ ion from the solution, which causes more of the NiS to dissolve.
Work Step by Step
NiS is more soluble in ammonia because the formation of the [Ni(NH$_{3}$)$_{6}$]$^{2+}$ complex ion is highly favorable. The formation removes Ni$^{2+}$ ion from the solution, which causes more of the NiS to dissolve.
