(a) $[\mathrm{Fe}(\mathrm{H}_2\mathrm{O})_6]^{3+}$Since $\mathrm{H}_2\mathrm{O}$ is a weak field ligand it cannot participate in pairing of electrons. Therefore, the number of unpaired electrons is 5 . $\mu=\sqrt{n(n+2)}=\sqrt{5(5+2)}$ $=\sqrt{35}=5.92\,\mathrm{BM}.$ Thus, it is strongly paramagnetic (due to presence of unpaired electrons). In $[\mathrm{Fe}(\mathrm{H}_2\mathrm{O})_6]^{3+}$, outer d-orbitals are used in hybridization and it is high spin or spin free complex. (b) Geometrical isomer of $[\mathrm{Pt}(\mathrm{en})_2\mathrm{Cl}_2]^{2+}$ which is optically active,