Paragraph: An aqueous solution of metal ion M 1 reacts separately...

Paragraph: An aqueous solution of metal ion $\mathbf{M} 1$ reacts separately with reagents $\mathbf{Q}$ and $\mathbf{R}$ in excess to give tetrahedral and square planar complexes, respectively. An aqueous solution of another metal ion M2 always forms tetrahedral complexes with these reagents. Aqueous solution of $\mathbf{M} 2$ on reaction with reagent $\mathbf{S}$ gives white precipitate which dissolves in excess of $\mathbf{S}$. The reactions are summarized in the scheme given below:

Question: Reagent $\mathbf{S}$ is