The spin-only magnetic moment value for the complex

$\left[\mathrm{Co}(\mathrm{CN})_{6}\right]^{4-}$

$x+6 \times(-1)=-4$

$\mathrm{Co}^{2+}:[\mathrm{Ar}] 3 \mathrm{~d}^{7}$

and $\mathrm{CN}^{-}$is a strong field ligand which can pair electron of central atom.

It has one unpaired electron (n) in 4d-subshell.

So spin only magnetic moment $(\mu)=$

$\sqrt{\mathrm{n}(\mathrm{n}+2)}$ B.M

where $\mathrm{n}=$ number of unpaired electrons.

$\mu=\sqrt{3} \quad$ B.M