The magnetic moment (spin only) of ${\left[{\mathrm{NiCl}}_4\right]}^{2-}$ is

${\left[{\mathrm{NiCl}}_4\right]}^{2-}$; oxidation number of Ni, x - 4 = -2

$\therefore \mathrm{x}=+2,{\mathrm{Ni}}_{\left(28\right)}\left[\mathrm{Ar}\right]3{\mathrm{d}}^8,4{\mathrm{s}}^2$

sp³-hybrid orbitals, tetrahedral

Since, $C{l}^{-}$ is a weak ligand, thus unpaired electrons are not paired. Lone pairs from 4 $C{l}^{-}$ are accommodated in four sp³
hybrid orbitals.
n = unpaired electron = 2, paramagnetic
Magnetic moment (spin only)

$=\sqrt{\mathrm{n}\left(\mathrm{n}+2\right)}\mathrm{BM}=\sqrt{8}=2.828\mathrm{BM}$