IUPAC name of the complex, ${\mathrm{Na}}_3\left[\mathrm{Cr}{\left(\mathrm{CN}\right)}_6\right]$ is

${\mathrm{CN}}^{-}$…cyanido or cyano(preferably cyanido)…uninegative ligand ;

Oxidation state of chromium is “+3” ;

Counter ion is sodium ion and while naming the counter ion  prefixes like di, tri…should not be used;

Complex ion is an anion, it should be written after name of cation;

While naming the complex, ligands are named first (in alphabetical order with proper prefixes) followed by metal ion with its oxidation state as roman number in parenthesis;

While naming the metal ion of an anionic complex the suffix “ate” is used;

Na₃[Cr(CN)₆] … Sodium hexacyanidochromate(III)