Question

The reaction of nitroprusside anion with sulphide ion gives purple colouration due to the formation of:
A. the tetranionic complex of iron (II) coordinating to one $\text{NO}{{\text{S}}^{-}}$ ion
B. the dianionic complex of iron (II) coordinating to one $\text{NC}{{\text{S}}^{-}}$ ion
C. the tri anionic complex of iron (III) coordinating to one $\text{NO}{{\text{S}}^{-}}$ion
D. the tetranionic complex of iron (III) coordinating to one $\text{NC}{{\text{S}}^{-}}$ ion

Answer 1

Hint: The colour of the complex depends on the ligand which makes the bond with the central atom. That means the different effect on energy is observed with different ligands and also they have a different wavelength.

Complete step by step answer:
- In the given, we have to tell the colour of complex which is formed when nitroprusside anion reacts with the sulphide ion.
- So, we have to write the correct balanced chemical reaction with a product that will be formed.
- As we know that the molecular formula of sodium nitroprusside is \[\text{N}{{\text{a}}_{2}}\left( \text{Fe(CN}{{\text{)}}_{5}}\text{NO} \right)\] whereas the molecular formula of sodium sulphide is $\text{N}{{\text{a}}_{2}}\text{S}$.

- So, now we have to write the reaction i.e.
\[\text{N}{{\text{a}}_{2}}\left( \text{Fe(CN}{{\text{)}}_{5}}\text{NO} \right)\text{ + N}{{\text{a}}_{2}}\text{S }\to \text{ N}{{\text{a}}_{4}}\left( \text{Fe(CN}{{\text{)}}_{5}}\text{NOS} \right)\]
- So, here the sulphide ion will join with nitrogen and oxygen to form nitroso sulfide i.e. \[\text{NO}{{\text{S}}^{-}}\].

- The ion formed after the reaction will be:
${{\left( \text{F}{{\text{e}}^{+2}}(\text{CN)}_{5}^{-5}\text{NOS} \right)}^{-4}}$ and the ion formed is tetra anionic complex of iron (II) with one nitroso sulfide which has purple or violet colour.
- As we can see that the total ion present on the complex is 4 that’s why it is named as tetra, anionic is used for the negative charge.
-Here, the oxidation state of iron is +2 that’s why it is written as (II) as we can see the charge +2 is present on iron.
So, the correct answer is “Option A”.

Note: The colour of any complexity can be easily determined with the help of Crystal field theory. Such as it is observed that the elements with full filled d - orbital or which have no electron in the d - orbital are colourless.