KCN is highly poisonous while `K_(4)[Fe(CN)_(6)]` is not. Why ?

Explanation

KCN ionizes to give `CN^(-)` ions which combine with the protein (cytochrome 'c') present in the blood and thereby block the oxygen binding site of the enzyme resulting into death
`KCNrarrK^(+)+CN^(-)`
`K_(4)[Fe(CN)_(6)]` ionizes as
`K_(4)[Fe(CN)_(6)]rarr4 K^(+)+[Fe(CN)_(6)]^(4-)`
The complex ion `[Fe(CN)_(6)]^(4-)` does not ionize to give `CN^(-)`ions. Hence, it is not toxic.