A colourless water-soluble solid ‘X’ on heating gives equimolar quantities of Y and Z. Y gives white dense fumes with $HCl$  and $Z$ also gives similar reaction with $N{H}_3$.  Y gives brown precipitate with Nessler’s reagent and Z gives white precipitate with nitrates of  $A{g}^{+},P{b}^{2+}$ and $H{g}^{+}$.  $\text{'}X\text{'}$ is :