\(Co = 6\);
\(O. N. = x + 5 \times (0) + 1 \times (-2) + 1\times (-1) = 0\)
\(\therefore \) \(x = + 3\) ; electronic configuration of
\(Co^{3+}[Ar]\, 3d^64s^0\) hence number of \(d\) electrons is \(6\). All \(d-\) electrons are paired due to strong ligand hence unpaired electron is zero.
$(i)\, [CoCl_3 (NH_3)_3]$ $(ii)\, [Co(en)_3]Cl_3$
$(iii)\, [Co(C_2O_4)_2(NH_3)_2]^-$ $(iv)\, [CrCl_2(NH_3)_2(en)]^+$