Question

In $N{{a}_{2}}[Fe{{(CN)}_{5}}NO]$, sodium nitroprusside:
(A) The oxidation state of Fe is +2.
(B) This has NO+ as a ligand.
(C) Both of the above
(D) None of the above

Answer 1

Remember that nitric oxide is a non-innocent ligand i.e. a ligand in a metal complex where its oxidation state is unclear. This concept results in the assumption that redox reactions in metal complexes are either localized to their metals or ligands, which is a useful simplification of the overarching concept.

Complete step by step answer:
Let us now analyse Sodium Nitroprusside as a compound to help us answer this question.
Sodium Nitroprusside is an inorganic compound with the formula, $N{{a}_{2}}[Fe{{(CN)}_{5}}NO]$ which is usually found in its dihydrate form, $N{{a}_{2}}[Fe{{(CN)}_{5}}NO]\cdot 2{{H}_{2}}O$. It is a red-coloured sodium salt which dissolves in either water or ethanol to give solutions which contain the complex dianion ${{[Fe{{(CN)}_{5}}NO]}^{2-}}$.

We observe that Nitroprusside is a complex anion which possesses an octahedral iron(III) centre (since Iron’s oxidation state in this compound is +3) which is surrounded by five tightly bound cyanide ligands and one linear nitric oxide ligand with its bond angle Fe-N-O angle being ${{176.2}^{\circ }}$ according to its structure which VSEPR theory.

Now, even though nitric oxide is a non-innocent ligand, the linear Fe-N-O angle results in the relatively short N-O distance of 113 pm and the subsequently high stretching frequency of $1947c{{m}^{1-}}$ , the complex is thus found to be containing an $N{{O}^{+}}$ ligand. Consequently, iron is assigned an oxidation state of +2; as a result of which the iron centre has a diamagnetic low-spin d6 electron configuration but a paramagnetic long-lived metastable state has been observed by EPR spectroscopy.

Thus, we can safely conclude that the answer to this question is (c).

Note: This compound decomposes to form sodium ferrous ferrocyanide, sodium ferrocyanide, nitric oxide, and cyanogen at about 450 °C. It decomposes in aqueous acid to liberate hydrocyanic acid (HCN) and if shielded from light, the concentrated solution is stable for more than two years at room temperature.