The pair having the same magnetic moment is : [At. No.: $\mathrm{Cr}=24, \mathrm{Mn}=25, \mathrm{Fe}=26, \mathrm{Co}=27$ ]

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

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

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

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

JEE Main · 2016 · 03 Apr 2016 · Offline · Coordination Compounds

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The pair having the same magnetic moment is :
[At. No.:

Cr = 24, Mn = 25, Fe = 26, Co = 27

]

(A) $[Cr (H_{2} O)_{6}]^{2 +}$ and $[Fe (H_{2} O)_{6}]^{2 +}$ ✓ Correct
(B) $[Mn (H_{2} O)_{6}]^{2 +}$ and $[Cr (H_{2} O)_{6}]^{2 +}$
(C) $[CoCl_{4}]^{2 -}$ and $[Fe (H_{2} O)_{6}]^{2 +}$
(D) $[Cr (H_{2} O)_{6}]^{2 +}$ and $[CoCl_{4}]^{2 -}$

Answer: (A)

Solution

Magnetic moment depends only on the number of unpaired electrons, through

μ = n (n + 2)

. With the weak-field aqua ligand both ions are high spin:

Cr X^{2 +}

d^{4}

t_{2 g}^{3} e_{g}^{1}

with four unpaired electrons, and

Fe X^{2 +}

d^{6}

t_{2 g}^{4} e_{g}^{2}

, also four unpaired. With the same

n = 4

[Cr (H X_{2} O) X_{6}]^{2 +}

and

[Fe (H X_{2} O) X_{6}]^{2 +}

have the same magnetic moment.

JEE Main · 2016 · 03 Apr 2016 · Offline · Single correct option

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JEE Main · 2016 · 03 Apr 2016 · Offline · Coordination Compounds

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