Chemistry - Coordination Chemistry Question with Solution | TestHub

With the strong-field ${\mathrm{CN}}^{-}$ ligand, all four complexes are low-spin octahedral:

${\left[\mathrm{Cr}(\mathrm{CN}{)}_6\right]}^{3-}$: ${\mathrm{d}}^3$ - 3 unpaired electrons

${\left[\mathrm{Mn}(\mathrm{CN}{)}_6\right]}^{3-}$: ${\mathrm{d}}^4,t_{2g}^4$ - 2 unpaired electrons

${\left[\mathrm{Fe}(\mathrm{CN}{)}_6\right]}^{3-}$: ${\mathrm{d}}^5,t_{2g}^5$ - 1 unpaired electron

${\left[\mathrm{Co}(\mathrm{CN}{)}_6\right]}^{3-}$: ${\mathrm{d}}^6,t_{2g}^6$ - 0 unpaired electrons (diamagnetic)

The cobalt complex shows the lowest paramagnetic behaviour. Answer: (D)