Chemistry - Coordination Chemistry Question with Solution | TestHub

1. For $[{\mathrm{MnCl}}_4{]}^{2-}$:

Manganese (Mn) atomic number is 25.

Let the oxidation state of Mn be $x$.

$x+4(-1)=-2$

$x-4=-2$

$x=+2$

Electronic configuration of Mn: $[\mathrm{Ar}]3d^{5}4s^2$

Electronic configuration of ${\mathrm{Mn}}^{2+}$: $[\mathrm{Ar}]3d^5$

${\mathrm{Cl}}^{-}$ is a weak field ligand. Therefore, no pairing of electrons occurs.

Number of unpaired electrons ($n$) = 5.

Magnetic moment $\mu =\sqrt{5(5+2)}=\sqrt{35}\approx 5.92$ B.M.

2. For $[{\mathrm{CoCl}}_4{]}^{2-}$:

Cobalt (Co) atomic number is 27.

Let the oxidation state of Co be $x$.

$x+4(-1)=-2$

$x-4=-2$

$x=+2$

Electronic configuration of Co: $[\mathrm{Ar}]3d^{7}4s^2$

Electronic configuration of ${\mathrm{Co}}^{2+}$: $[\mathrm{Ar}]3d^7$

${\mathrm{Cl}}^{-}$ is a weak field ligand. Therefore, no pairing of electrons occurs.

Number of unpaired electrons ($n$) = 3.

Magnetic moment $\mu =\sqrt{3(3+2)}=\sqrt{15}\approx 3.87$ B.M.

3. For $[\mathrm{Fe}(\mathrm{CN}{)}_6{]}^{4-}$:

Iron (Fe) atomic number is 26.

Let the oxidation state of Fe be $x$.

$x+6(-1)=-4$

$x-6=-4$

$x=+2$

Electronic configuration of Fe: $[\mathrm{Ar}]3d^{6}4s^2$

Electronic configuration of ${\mathrm{Fe}}^{2+}$: $[\mathrm{Ar}]3d^6$

${\mathrm{CN}}^{-}$ is a strong field ligand. Therefore, pairing of electrons occurs.

In an octahedral field, the $3d^6$ configuration with a strong field ligand will have $t_{2g}^6{e}_g^0$.

Number of unpaired electrons ($n$) = 0.

Magnetic moment $\mu =\sqrt{0(0+2)}=0$ B.M.

Comparing the magnetic moments:

$[{\mathrm{MnCl}}_4{]}^{2-}$: 5.92 B.M.

$[{\mathrm{CoCl}}_4{]}^{2-}$: 3.87 B.M.

$[\mathrm{Fe}(\mathrm{CN}{)}_6{]}^{4-}$: 0 B.M.

Order of magnetic moments: $[{\mathrm{MnCl}}_4{]}^{2-}>[{\mathrm{CoCl}}_4{]}^{2-}>[\mathrm{Fe}(\mathrm{CN}{)}_6{]}^{4-}$

The final answer is $\boxed{A}$.