In nitroprusside ion, the iron and NO exist as Fe⁺⁺ and NO⁺ rather than Fe ⁺⁺⁺ and NO. These forms can be differentiated by

The nitroprusside ion is ${\left[\mathrm{Fe}(\mathrm{CN}{)}_6{\mathrm{NO}}^{+}\right]}^{2-}$ The magnetic moment measurements reveal the presence of 4 unpaired electrons in Fe which must be then in Fe⁺⁺ $\left(3{\mathrm{d}}^6\right)\text{ and not }{\mathrm{Fe}}^{+++}\left(3{\mathrm{d}}^5\right)$