When a metal rod $\mathbf{M}$ is dipped into an aqueous colourless concentrated solution of compound $\mathbf{N}$, the solution turns light blue. Addition of aqueous NaCl to the blue solution gives a white precipitate $\mathbf{O}$. Addition of aqueous $\mathrm{NH}_3$ dissolves $\mathbf{O}$ and gives an intense blue solution.

The compound N is