In $\left[ Co \left( NH _{3}\right)_{6}\right]^{3+}$,
oxidation state of $Co =+3$
and its coordination number is six.
So, ${ }_{27} Co =1 s^{2},\, 2 s^{2},\, 2 p^{6}, \,3 s^{2}\, 3 p^{6}\, 3 d^{7},\, 4 s^{2}$
$Co ^{3+}=1 s^{2}\, 2 s^{2} \,2 p^{6},\, 3 s^{2}\, 3 p^{6} \,3 d^{6}$
Thus, $\left[ Co \left( NH _{3}\right)_{6}\right]^{3+}$ shows inner orbital complex as well as diamagnetic in behaviour (due to absence of unpaired electron).
$\left[ Zn \left( NH _{3}\right)_{6}\right]^{2+} \rightarrow s p^{3} d^{2}$ hybridisation (outer) and diamagnetic.
$\left[ Cr \left( NH _{3}\right)_{6}\right]^{3+} \rightarrow d^{2} s p^{3}$ hybridisation (inner) and paramagnetic.
