Total possible linkage isomers of ${\mathrm{K}}_4\left[\mathrm{Fe}(\mathrm{CN}{)}_5\left({\mathrm{NO}}_2\right)\right]$

[M (AA) abcd]

$\begin{array}{r}a\text{ opp }b\to 1\\ a\text{ opp }c\to 1\\ a\text{ opp }d\to 1\\ b\text{ opp }c\to 1\\ b\text{ opp }d\to 1\\ \text{ copp }d\to \frac{1}{6}\\ \mathrm{\_}\\ \end{array}$

$$\begin{gathered} 6 \times 2 \times 2 = 24 \\ \Downarrow \Downarrow \\ \mathrm{O}-\mathrm{donor} \mathrm{All} \mathrm{O}.\mathrm{A}. \\ \mathrm{form} {\mathrm{NO}}_2 \end{gathered}$$