Question

Among the following which one has the highest paramagnetism?

• A. $[\mathrm{Cr}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6{]}^{3+}$
• B. $[\mathrm{Fe}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6{]}^{2+}$
• C. $[\mathrm{Cu}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6{]}^{2+}$
• D. $[\mathrm{Zn}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6{]}^{2+}$

Answer 1

The correct option is B

$[\mathrm{Fe}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6{]}^{2+}$

The explanation for the correct option

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

Paramagnetism

• Paramagnetism is the ability of certain materials to be weakly attracted to the magnetic field.
• Paramagnetism is exhibited by molecules containing unpaired electrons.
• The paramagnetism property of a complex increases with the larger number of unpaired electrons.

• The electronic configuration of ${\mathrm{Fe}}^{2+}$ is $\left[\mathrm{Ar}\right]3{\mathrm{d}}^6$
• ${\mathrm{H}}_2\mathrm{O}$ is a weak field ligand it does not pair up unpaired electrons.
• The number of unpaired electrons in $[\mathrm{Fe}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6{]}^{2+}$ is $4$
• As the number of unpaired electrons is higher, the paramagnetic property of the complex is also higher.

Hence Option (B) $\mathbf{[}\mathbf{Fe}{\mathbf{\left(}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}\mathbf{\right)}}_{\mathbf{6}}{\mathbf{]}}^{\mathbf{2}\mathbf{+}}$is correct

The explanation for the incorrect options

(A) $\mathbf{[}\mathbf{Cr}{\mathbf{\left(}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}\mathbf{\right)}}_{\mathbf{6}}{\mathbf{]}}^{\mathbf{3}\mathbf{+}}$

• The electronic configuration of ${\mathrm{Cr}}^{3+}$ is $\left[\mathrm{Ar}\right]3{\mathrm{d}}^3$
• ${\mathrm{H}}_2\mathrm{O}$ is a weak field ligand it does not pair up unpaired electrons.
• The number of unpaired electrons in $[\mathrm{Cr}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6{]}^{3+}$ is $3$
• As the number of unpaired electrons is lower the paramagnetism property of the complex is also lower.

(C) $\mathbf{[}\mathbf{Cu}{\mathbf{\left(}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}\mathbf{\right)}}_{\mathbf{6}}{\mathbf{]}}^{\mathbf{2}\mathbf{+}}$

• The electronic configuration of ${\mathrm{Cu}}^{2+}$ is $\left[\mathrm{Ar}\right]3{\mathrm{d}}^9$
• ${\mathrm{H}}_2\mathrm{O}$ is a weak field ligand it does not pair up unpaired electrons.
• The number of unpaired electrons in $[\mathrm{Cu}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6{]}^{2+}$ is $1$
• As the number of unpaired electrons is lower the paramagnetism property of the complex is also lower.

(D) $\mathbf{[}\mathbf{Zn}{\mathbf{\left(}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}\mathbf{\right)}}_{\mathbf{6}}{\mathbf{]}}^{\mathbf{2}\mathbf{+}}$

• The electronic configuration of ${\mathrm{Zn}}^{2+}$ is $\left[\mathrm{Ar}\right]3{\mathrm{d}}^{10}$
• ${\mathrm{H}}_2\mathrm{O}$ is a weak field ligand it does not pair up unpaired electrons.
• The number of unpaired electrons in $[\mathrm{Zn}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6{]}^{2+}$ is $0$
• There are no unpaired electrons. Hence this complex is diamagnetic

Therefore, Option (B) $\mathbf{[}\mathbf{Fe}{\mathbf{\left(}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}\mathbf{\right)}}_{\mathbf{6}}{\mathbf{]}}^{\mathbf{2}\mathbf{+}}$ is correct