An orange solid $\mathrm{A}$ on heating gives a green residue $\mathrm{B},$ a colourless gas $\mathrm{C}$ and water vapours. The dry gas $\mathrm{C}$ upon pasing over heated $\mathrm{Mg}$ gave a white solid $\mathrm{D}$ which upon subsequent reaction with water gave a gas $\mathrm{E}$ that gave dense white fumes with $\mathrm{HCl}$.

Identify $\mathrm{D}$