In [CO(H_2 O)_6]^ 2+ , there are three unpaired electrons present....

In $\left[\mathrm{Co}\left(\mathrm{H}_2 \mathrm{O}\right)_6\right]^{2+}$, there are three unpaired electrons present as, $\mu_{\mathrm{cal}}=\sqrt{n(n+1)} \mathrm{BM}=\sqrt{3(3+1)} \mathrm{BM}=3.87 \mathrm{BM}$ As, the $t_{2 g}$ orbital is unsymmetrically filled hence due to orbital motion contribution, magnetic moment goes with the value 4.40 BM