$\mathrm{CN}^{-}$ is a strong field ligand. This is due to the fact that
$\mathrm{CN}^{-}$ is a strong field ligand. This is due to the fact that
It can accept electron from metal species
It forms high spin complexes with metal species
It carries negative charge
It is a pseudohalide
Solution
Cyanide ion is strong field ligand because it is a pseudohalide ion, pseudohalide ions are stronger coordinating ligand & they have the ability to form $\sigma$ bond from pseudohalide to the metal and $\pi$ bond (from the metal to pseudohalide).