The chemical formula of sodium nitroprusside is $Na_2[Fe(CN)_5NO]$. In this complex, the NO ligand is present as the nitrosonium ion ($NO^+$), and $CN$ is the cyanide ion ($CN^-$). Let the oxidation state of Fe be $x$. Charge on the coordination sphere $[Fe(CN)_5NO]^{2-}$ is $-2$. So, $x + 5(-1) + (+1) = -2 \implies x - 4 = -2 \implies x = +2$. Therefore, the central iron atom is in the $+2$ oxidation state. According to IUPAC nomenclature rules, ligands are named in alphabetical order: five cyanido ligands ('pentacyanido') and one nitrosyl ligand ('nitrosyl'). The anionic complex ends with the suffix '-ate', making the central metal 'ferrate(II)'. Thus, the complete IUPAC name is Sodium pentacyanidonitrosyl ferrate(II).