An inorganic salt containing sulphur gives a characteristic purple...

The reaction of sulphide ions with sodium nitroprusside yields a purple coloured complex, $\mathrm{Na}_4\left[\mathrm{Fe}(\mathrm{CN})_5 \mathrm{NOS}\right]$.

The complex ion is \left[\mathrm{Fe}(\mathrm{CN})_5 \mathrm{NOS}\right]^{4-}$.<br><br>In this complex, the ligands are five cyanide ions ($\mathrm{CN}^{-}$) and one$\mathrm{NOS}^{-}$ion. The$\mathrm{NOS}^{-}$ligand is formed by the attack of$\mathrm{S}^{2-}$on the$\mathrm{NO}^{+}$ligand of the original nitroprusside complex, resulting in a net charge of$-1$for the$\mathrm{NOS}$group.<br><br>Let the oxidation state of iron ($\mathrm{Fe}$) be$x$.<br>The sum of oxidation states is equal to the net charge on the complex ion:<br>$x + 5(-1) + 1(-1) = -4$<br>$x - 6 = -4$<br>$x = +2$<br><br>Therefore, the oxidation state of the central metal atom is$+2$.<br><br>Answer:$+2