An inorganic Lewis acid (X) fumes in moist air, and the intensity of fumes increases when a rod dipped in $\mathrm{NH_4OH}$ is brought near it. An acidic solution of (X) on addition of $\mathrm{NH_4Cl}$ and $\mathrm{NH_4OH}$ gives a precipitate which dissolves in NaOH solution. A solution of (X) does not give precipitate with $\mathrm{H_2S}$. Hence, compound (X) is: