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Q. The pair in which both the species have the same magnetic moment (spin only) is :

JEE MainJEE Main 2020Coordination Compounds

A

$\left[ Mn \left( H _{2} O \right)_{6}\right]^{2+}$ and $\left[ Cr \left( H _{2} O \right)\right]^{2+}$

12%

B

$\left[ Cr \left( H _{2} O \right)_{6}\right]^{2+}$ and $\left[ CoCl _{4}\right]^{2-}$

16%

C

$\left[ Cr \left( H _{2} O \right)_{6}\right]^{2+}$ and $\left[ Fe \left( H _{2} O \right)_{6}\right]^{2+}$

59%

D

$\left[ Co ( OH )_{4}\right]^{2-}$ and $\left[ Fe \left( NH _{3}\right)_{6}\right]^{2+}$

13%

Solution:

Complex $e^-$ configuration no. of unpaired $e^-$

$[Mn(H_2O)_6]^{2+}$
WFL 5

$[Cr(H_2O)_6]^{2+}$
WFL 4

$[COCl_4]^{2-}$
Tetrahedral 3

$[Fe(H_2O)_6]^{2+}$
WFL 4

$[Co(OH)_4]^{2-}$
WFL 3

Tetrahedral
$[Fe(NH_3)_6]^{2+}$ 4

Thus complex $\left[ Cr \left( H _{2} O \right)_{6}\right]^{2+}$ and $\left[ Fe \left( H _{2} O \right)_{6}\right]^{2+}$ have same no. of unpaired $e ^{-}$ and hence same magnetic moment (spin only).