(a) ${\left[\mathrm{Fe}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{3+}$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 .
$\mathit{\mu }=\sqrt{\mathit{n}\left(\mathit{n}+2\right)}=\sqrt{5\left(5+2\right)}$ $=\sqrt{35}=5.92\mathrm{BM}.$
Thus, it is strongly paramagnetic (due to presence of unpaired electrons).
In ${\left[\mathrm{Fe}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{3+}$, outer d-orbitals are used in hybridization and it is high spin or spin free complex.
(b) Geometrical isomer of ${\left[\mathrm{Pt}{\left(\mathrm{en}\right)}_2{\mathrm{Cl}}_2\right]}^{2+}$ which is optically active,