According to IUPAC nomenclature sodium nitroprusside is named as

IUPAC name of sodium nitroprusside $N{a}_2\left[Fe{\left(CN\right)}_5.NO\right]$ is sodium pentacyanoni trosyl ferrate (III) because in it NO is neutral ligand and the oxidation number of Fe is + 3. Which is calculated as $N{a}_2\left[Fe{\left(CN\right)}_{5}NO\right]$
$2\times \left(+1\right)+x+5\times \left(-1\right)+1\times 0=0$
[where x= oxidation state of Fe]
$2+x-5=0$
$x-3=0$
$x=+3$