The primary valency of iron in K₄[Fe(CN)₆] is satisfied by

Primary valency is the number of negative ions which is required to satisfy the charge on the central metal ion. They are also called as oxidation states. In the complex, the oxidation state of Fe (x) in K₄[Fe(CN)₆] is:

$$\begin{gathered} \mathrm{x}+4(+1)+6(-1)=0 \\ \mathrm{x}=+2 \end{gathered}$$

So, the primary valency is 2 CN^- ions.