The complex K₃[Fe(CN)₆] should have a spin only magnetic moment of

${\mathrm{K}}_3\left[\stackrel{+3}{\mathrm{Fe}}{\left(\mathrm{CN}\right)}_6\right]$

Fe⁺³: 4s⁰3d⁵

no. of unpaired e^- (n)=1 ($\because$CN is a strong field ligand)

Spin only magnetic moment= $\sqrt{\mathrm{n}(\mathrm{n}+2) }=\sqrt{1(1+2)}=\sqrt{3}\mathrm{BM}$