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  4. The correct order of the spin-only magnetic moments of the following complexes is: (I) [Cr(H2O)6]Br2 (II) Na4[Fe(CN)6] (III) Na3[Fe(C2O4)3] (Δ0 > P) (IV) (Et4N)2[CoCl4]

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Q. The correct order of the spin-only magnetic moments of the following complexes is :
$(I)\, [Cr(H_2O)_6]Br_2$
$(II)\, Na_4[Fe(CN)_6]$
$(III)\, Na_3[Fe(C_2O_4)_3] (\Delta_0 > P)$
$(IV)\, (Et_4N)_2[CoCl_4]$

JEE MainJEE Main 2020Coordination Compounds

Solution:

$I \left[Cr\left(H_{2}O\right)_{6}\right]^{2+}$
$Cr^{+2} \Rightarrow \left[Ar\right]3d^{4}$
$H_{2}O\to$ Weak field ligand
Unpaired $e^{-}=4$
Magnetic moment=$\sqrt{24} BM$
$=4.89 BM$
II $\left[Fe\left(CN\right)_{6}\right]^{4-}$
$Fe^{+2} \Rightarrow \left[Ar\right]3d^{6}$
$CN^{-} \to $ Strong field ligand
Unpaired $e^{-}=0$
Magnetic moment = 0 BM=0 BM
III $\left[Fe\left(C_{2}O_{4}\right)_{3}\right]^{3-}$
$Fe^{+3} \Rightarrow \left[Ar\right]3d^{5}$
$As\, \Delta_{0} >\,P$
$Unpaired \, e^{-}=1$
$Magnetic\,moment=\sqrt{3} BM =1.73 BM$
$IV \left(Et_{4}N\right)^{+} \left[CoCl_{4}\right]^{2-}$
$Co^{+2} \Rightarrow \left[Ar\right]3d^{7}$
$\left[CoCl_{4}\right]^{-2}=$
$Unpaired \,electrons = 3$
$Magnetic \,moment =\sqrt{15} BM =3.87 BM$
$Hence\, order \,of \,magnetic\, moment\,is\,$
$I > IV > III > II$

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