Which one of the following is an inner orbital complex as well as diamagnetic in behaviour ?
$$\left( {{\text{At}}{\text{. no}}{\text{. of}}\,\,Zn = 30,Cr = 24,Co = 27,Ni = 28} \right)$$

Solution :
In $${\left[ {Co{{\left( {N{H_3}} \right)}_6}} \right]^{3 + }},$$   oxidation state of $$Co = + 3$$   and its coordination number is six.
$$\eqalign{ & _{27}Co = 1{s^2},2{s^2}2{p^6},3{s^2}3{p^6}3{d^7},4{s^2} \cr & C{o^{3 + }} = 1{s^2},2{s^2}2{p^6},3{s^2}3{p^6}3{d^6} \cr} $$

$${\left[ {Co{{\left( {N{H_3}} \right)}_6}} \right]^{3 + }}$$   is an inner orbital complex as well as diamagnetic in behaviour ( due to absence of unpaired electrons ).
$${\left[ {Zn{{\left( {N{H_3}} \right)}_6}} \right]^{2 + }} \to s{p^3}{d^2}$$     hybridisation ( outer orbital complex and diamagnetic in nature ).
$${\left[ {Cr{{\left( {N{H_3}} \right)}_6}} \right]^{3 + }} \to {d^2}s{p^3}$$     hybridisation ( inner orbital complex and paramagnetic in nature ).