Consider the complex ion $[\text{Fe}({\text{H}}_2\text{O}{)}_5(\text{NO}){]}^{2+}$. Upon formation of the brown ring complex, the oxidation state of iron and the effective magnetic moment in Bohr magnetons are respectively:

Solution

1. The brown ring complex is $[\text{Fe}({\text{H}}_2\text{O}{)}_5(\text{NO})]S{O}_4$.
2. The NO ligand exists as ${\text{NO}}^{+}$ and the iron center is ${\text{Fe}}^{+}$.
3. The electronic configuration of ${\text{Fe}}^{+}$ is $3d^7$, which has three unpaired electrons in a weak field.
4. The magnetic moment is $$\begin{array}{rl}\mu & =\sqrt{n(n+2)}\\ & =\sqrt{3(5)}\\ & =\sqrt{15}\approx 3.87\text{ BM}\end{array}$$.
5. Hence the answer is (A).