KEAM 2011 Physics and Chemistry Paper

(i) In $[\mathrm{CoF}_6]^{3-}$ ion, the oxidation state of cobalt is +3 . $\mathrm{Co}^{3+}$ ionFormation of $[\mathrm{CoF}_6]^{3+}$ Here, $\mathrm{F}^{-}$ ion provides a weak ligand field and is unable to pair up the unpaired electrons of the $3d$ -orbitals. Thus, it is highly paramagnetic. (ii) In $[\mathrm{Co}(\mathrm{NH}_3)_6]^{3+}$ ion, $\mathrm{NH}_3$ provides a strong field ligand and the electrons of metal are made to pair up, so the complex will be diamagnetic. Formation of [Co (NH $_{3})6]^{3+}$ (iii) Similarly in $[\mathrm{Ni}(\mathrm{NH}_3)_4]^{2+}, \mathrm{NH}_3$ is a strong field ligand and the complex is diamagnetic. (iv) $[\mathrm{Ni}(\mathrm{CN})_4]^{2-}$ has $\mathrm{Ni}$ in the +2 oxidation state and $d s p^{2}$ hybridisation. Thus it is also diamagnetic. (v) In [Fe(CN) $_{6}]^{4-}$ ions, oxidation state of Fe is +2 and $\mathrm{CN}^{-}$ ligand is a strong field ligand. Thus the resulting complex ion involves $d^{2} s p^{3}$ hybridisation and diamagnetic as it does not contain any unpaired electrons.∵ Magnetic moment $=\sqrt{n(n+2)}$ where, $n=$ no. of unpaired electrons $\therefore [\mathrm{CoF}_6]^{3-}$ has maximum magnetic moment.