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  4. Which of the following cyano complexes would exhibit the lowest value of paramagnetic bshaviour, ?

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Q. Which of the following cyano complexes would exhibit the lowest value of paramagnetic bshaviour, ?

AIEEEAIEEE 2005The d-and f-Block Elements

Solution:

Outer electronic configuration of $M^{3+}$ ions are :
$Cr^{3+} : 3d^3, Mn^{3+} : 3d^4, Fe^{3+} : 3d^5$ and $Co^{3+} : 3d^6$ ligand has a tendency to pair up $AsCN^-\, (n - 1)d$ electrons for $d^2sp^3$ hybridisation number of unpaired electrons in these complexes will be
$ [Cr(CN)_6]^{3-}$ 3 unpaired electrons (No pairing up)
$[Mn(CN)_6]^{3-}$ 2 unpaired electrons
$ [Fe(CN)_6]^{3-}$ 1 unpaired electron
$ [Co(CN)_6]^{3-}$ No unpaired electro
Therefore, complex expected to have lowest paramagnetic behaviour (with no unpaired electron) is $[Co(CN)_6]^{3-}$