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.
(પ.ક્ર.: $Mn\, = 25, Co\, = 27, Ni\, = 28, Zn\, = 30$)
${\left[\mathrm{Fe}\left(\mathrm{H}_2 \mathrm{O}\right)_6\right]^{2+},\left[\mathrm{Co}\left(\mathrm{H}_2 \mathrm{O}\right)_6\right]^{2+},\left[\mathrm{Co}\left(\mathrm{H}_2 \mathrm{O}\right)_6\right]^{3+},}$
${\left[\mathrm{Cu}\left(\mathrm{H}_2 \mathrm{O}\right)_6\right]^{2+},\left[\mathrm{Cr}\left(\mathrm{H}_2 \mathrm{O}\right)_6\right]^{2+}}$
$(1)\ [CO(NH_3)_5(NO_2)]Cl$ અને $[CO(NH_3)_5(ONO)]Cl_2$ લીન્કેજ
$(2)\ [Cu(NH_3)_4][PtCl_4]$ અને $[Pt(NH_3)_4][CuCl_4]$ સવર્ગ
$(3)\ [Pt(NH_3)_4Cl_2]Br_2$ અને $[Pt(NH_3)_4Br_2)]Cl_2$ આયનીકરણ