$\mu_1=\sqrt{3(3+2)}=\sqrt{15}\,BM$
Spin magnetic moment of $\left[ Cr \left( H _2 O \right)_6\right]^{3+}\left( t _{2 g }^3 e _{ g }^0\right)$
$\mu_2=\sqrt{3(3+2)}=\sqrt{15}\,BM$
$\frac{\mu_1}{\mu_2}=\frac{\sqrt{15}}{\sqrt{15}}=1$
$(I)\, [Ni(CN)_4]^{2-}\,\,\,(II)\, [NiCl_4]^{2-}\,\,\,(III)\,Ni(CO_4)\,\,\,$
$(IV)\, [Ni(H_2O)_6 ]^{2+}$