The spin only magnetic moment of cobalt in the compound $Hg\left[Co{\left(SCN\right)}_4\right]$ is _____ ?

In the complex $Hg\left[Co{\left(SCN\right)}_4\right]$, Co is in +2 oxidation state.

Electronic configuration of Co(II) :$Ar4s^{0}3d^7$

Now thiocyanato is a weak field ligand and so there will be no back pairing.

Spin only magnetic moment is given as ${\mu }_m=\sqrt{n(n+2)} BM$

Putting n=3, we get ${\mu }_m=\sqrt{15}BM$