Magnetic behaviour of complex compounds


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> Which one of the following cyanide complexes would exhibit the lowest value of paramagnetic behaviour?
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1. $\ce{[Cr(CN)6]^{3-}}$
2. $\ce{[Mn(CN)6]^{3-}}$
3. $\ce{[Fe(CN)6]^{3-}}$
4. $\ce{[Co(CN)6]^{3-}}$


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:


[![hybridisation of complexes-1](https://i.stack.imgur.com/uSfDu.jpg)](https://i.stack.imgur.com/uSfDu.jpg)


[![hybridisation of complexes-2](https://i.stack.imgur.com/kxKoI.jpg)](https://i.stack.imgur.com/kxKoI.jpg)
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


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This is because in Co+3 there is no unpaired electrons .i.e all paired up (due to the strong ligand's effect) due to which it will be diamagnetic in nature and hence it will have the least paramagnetic behaviour.


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