Chemistry - Coordination Chemistry Question with Solution | TestHub

(P) ${\mathrm{Ni}}^{2+}(\mathrm{aq})$ is octahedral and paramagnetic (${\mathrm{d}}^8$, 2 unpaired); with dmg it forms $\left[\mathrm{Ni}(\mathrm{dmg}{)}_2\right]$, square planar and diamagnetic - both the magnetic moment and the crystal field splitting pattern change. $✓$

(Q) ${\mathrm{Fe}}^{2+}\to {\left[\mathrm{Fe}(\mathrm{CN}{)}_6\right]}^{4-}$: geometry stays octahedral (${\mathrm{d}}^6$ high spin $\to$ low spin) - only the magnetic moment changes. $\times$

(R) ${\mathrm{Co}}^{2+}\to {\left[{\mathrm{CoCl}}_4\right]}^{2-}$: octahedral $\to$ tetrahedral, but ${\mathrm{d}}^7$ keeps 3 unpaired electrons in both geometries - the spin-only moment is unchanged. $\times$

(S) ${\mathrm{Co}}^{2+}\to {\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_6\right]}^{3+}$: geometry stays octahedral; only the moment changes. $\times$

Only (P) - Answer: (A)