For a diamagnetic complex, there should not be any unpaired electron in the valence shell of central metral. In \(\mathrm{K}_3\left[\mathrm{Fe}(\mathrm{CN})_6\right]\), Fe (III) has \(d^5\)-configuration (odd electrons), hence it is paramagnetic. In [Co( \(\left.\left.\mathrm{NH}_3\right)_6\right] \mathrm{Cl}_3\), Co (III) has \(d^6\) configuration in a strong ligand field, hence all the electrons are paired and the complex is diamagnetic. In \(\mathrm{Na}_3\left[\mathrm{Co}(\mathrm{ox})_3\right]\), \(\mathrm{Co}\) (III) has \(d^6\)-configuration and oxalate being a chelating ligand, very strong ligand and all the six electrons remains paired in lower \(t_{2 g}\) level, diamagnetic. In \(\left[\mathrm{Ni}\left(\mathrm{H}_2 \mathrm{O}\right)_6\right] \mathrm{Cl}_2, \mathrm{Ni}\) (II) has \(3 d^8\)-configuration and \(\mathrm{H}_2 \mathrm{O}\) is a weak ligand hence


In \(\mathrm{K}_2\left[\mathrm{Pt}(\mathrm{CN})_4\right]\), Pt (II) has \(d^8\)-configuration and \(\mathrm{CN}^{-}\)is a strong ligand, hence all the eight electrons are spin-paired. Therefore, complex is diamagnetic.
In \(\left[\mathrm{Zn}\left(\mathrm{H}_2 \mathrm{O}\right)_6\right]\left(\mathrm{NO}_3\right)_2, \mathrm{Zn}\) (II) has \({ }_3 \mathrm{~d}^{10}\) configuration spin paired, hence diamagnetic.