One unpaired electron causes magnetic moment of 1.1 BM. The magnetic moment of ${}_{26}\mathrm{Fe}^{2+}$ is

electronic configuration of $F{e}^{2+}:\left[Ar\right]3d^{6}4s^0$

$$\begin{gathered} Magnetic moment = \sqrt{n(n+2)} \\ = \sqrt{4(4+2)} \\ = 4.9BM \end{gathered}$$

number of unpaired electrons(n) = 4