Magnetic moment of the complex${\left[\mathrm{Fe}(\mathrm{CN}{)}_6\right]}^{-3}$approximately is

In ${\left[\mathrm{Fe}(\mathrm{CN}{)}_6\right]}^{-3}$

The O. State of Fe is +3 

Electronic configuration is  $\left[\mathrm{Ar}\right]3{\mathrm{d}}^5$

${\mathrm{CN}}^{-}$ is strong field ligand ,there will be the pairing of electrons and only one electron will be unpaired.

n =1 

Magnetic moment = $\sqrt{\mathrm{n}(\mathrm{n}+2)} =\sqrt{1(1+2) }=\sqrt{3}$ =1.732M