Magnetic behaviour of complex compounds Which one of the following cyanide complexes would exhibit the lowest value of paramagnetic behaviour?



$\ce{[Cr(CN)6]^{3-}}$
$\ce{[Mn(CN)6]^{3-}}$
$\ce{[Fe(CN)6]^{3-}}$
$\ce{[Co(CN)6]^{3-}}$


Here I determined central atom configuration and it is $\mathrm{3d^3}, \mathrm{3d^4}, \mathrm{3d^5}, \mathrm{3d^6}$ for each option in order. $\ce{CN-}$ is a strong-field ligand so electron will get paired hence option 2 and 4 will have least magnetic behaviour 

But only option 4 is correct. Why?
Magnetic behaviour of complex compounds are determined on the basis of magnetic moment, rather the extent of paramagnetism is measured in terms of the magnetic moment μ.  

μ = √n(n+2) BM , where n notates the number of unpaired electrons. 

First lets give a look towards the number of unpaired electron each of the central metal atom has:




Here the question is asking to determine the complex compound which shows the lowest paramagnetic behaviour. As Co^3+ has no unpaired electron, its μ = 0, hence it is diamagnetic, and hence shows shows lowest paramagnetic behaviour among the above complexes.
And best of luck for jee'17.
Me too appearing the exam.

Hope it helps