An inorganic salt containing sulphur gives a characteristic purple colouration when its sodium carbonate extract is treated with sodium nitroprusside. The oxidation state of the central metal atom in the newly formed purple complex is :

Step-by-step solution

The reaction of sulphide ions with sodium nitroprusside yields a purple coloured complex, Na ₄ [ Fe ( CN )₅ NOS ] . The complex ion is [ Fe ( CN )₅ NOS ]⁴⁻ . In this complex, the ligands are five cyanide ions ( CN ⁻ ) and one NOS ⁻ ion. The NOS ⁻ ligand is formed by the attack of S ²⁻ on the NO ⁺ ligand of the original nitroprusside complex, resulting in a net charge of -1 for the NOS group. Let the oxidation state of iron ( Fe ) be x . The sum of oxidation states is equal to the net charge on the complex ion: x +