A certain metal (X) is boiled in dilute HNO 3 to give a salt (Y) and an oxide of nitrogen…

A certain metal (X) is boiled in dilute HNO₃ to give a salt (Y) and an oxide of nitrogen (Z). An aqueous solution of (Y) with brine solution gives a white precipitate (P). When (Y) was treated with Na₂S₂O₃ solution a white precipitate (Q) was obtained which on standing turns to a black compound (R).
The compound (R) formed is