The electronic configuration of $Co ^{3+}$ ion
$=[A r]_{18} 3 d^{6}$
The splitting in octahedral field is shown below
$C o^{3+}=3 d^{6}\left(\right.$ or $\left.t_{2 g}^{6} e_{g}\right)$
$\therefore$ There is no unpaired electron, so it is diamagnetic in nature.
($A$ અને $B$ એકદંતીય લિગેન્ડ છે.)
$(i)$ $\left[ FeF _{6}\right]^{3-}$
$(ii)$ $\left[ Co \left( NH _{3}\right)_{6}\right]^{3+}$
$(iii)$ $\left[ NiCl _{4}\right]^{2-}$
$(iv)$ $\left[ Cu \left( NH _{3}\right)_{4}\right]^{2+}$