The calculated magnetic moments (spin only value) for species ${\left[{\mathrm{FeCl}}_4\right]}^{2-}$, ${\left[\mathrm{Co}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_3\right]}^{3-}$and ${{\mathrm{MnO}}_4}^{2-}$respectively are:

The correct option is C

$4.90\mathrm{BM},0\mathrm{BM}\mathrm{and}1.73\mathrm{BM}$

The correct option is option C

The explanation for the correct option:

1. The measurement of magnetic properties is frequently defined as the magnetic moment.
2. It is given by $\mathrm{\mu }=\sqrt{\mathrm{n}(\mathrm{n}+2)}$ where n is the number of unpaired electrons present in the central atom.
3. For calculating the magnetic moment we first need to find the oxidation state of the central atom, then if it has strong field ligands then bonding of electrons occurs and at last count the number of unpaired electrons present in the d-orbital.
4. For ${\left[{\mathrm{FeCl}}_4\right]}^{2-}$ the central atom is iron.
5. Let the oxidation number of iron be “x”, it is given by Charge on complex$=$Oxidation number of iron $+$$4\times$oxidation number of chlorine.
6. $-2=\mathrm{x}+\left(4\right)-1,\mathrm{x}=+2$ i.e. it will lose its two electron
7. Electronic configuration of ${\mathrm{Fe}}^{+2}=1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^{6}3{\mathrm{d}}^6$ also chlorine is a weak field ligand and thus no pairing will occur.
8. Magnetic moment$=\sqrt{4(4+2)}\Rightarrow \sqrt{24}\Rightarrow 4.9\mathrm{BM}$
9. For ${\left[\mathrm{Co}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_3\right]}^{3-}$the central atom is cobalt.
10. Let the oxidation number of cobalt be “x”, it is given by Charge on complex$=$the oxidation number of cobalt $+$$3\times$oxidation number of oxalate.
11. $-3=\mathrm{x}+(-2)\times 3,\mathrm{x}=+3$i.e. it will lose three electrons.
12. Electronic configuration of ${\mathrm{Co}}^{+3}=1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^{6}3{\mathrm{d}}^6$, since oxalate is a strong field ligand pairing will occur.
13. There are no unpaired electrons so the magnetic moment is zero.

1. For ${{\mathrm{MnO}}_4}^{2-}$manganese is the central atom.
2. Let the oxidation number of manganese be “x”, it is given by Charge on complex$=$the oxidation number of oxygen$\times 4$
3. $-2=\mathrm{x}+4(-2)$$,\mathrm{x}=+6$
4. Electronic configuration of ${\mathrm{Mn}}^{+6}=1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^{6}3{\mathrm{d}}^1$
5. Magnetic moment$=\sqrt{1(1+2)}\Rightarrow \sqrt{3}\Rightarrow 1.73\mathrm{BM}$

Therefore, the calculated magnetic moments (spin only value) for species ${\left[{\mathrm{FeCl}}_4\right]}^{2-}$, ${\left[\mathrm{Co}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_3\right]}^{3-}$and ${{\mathrm{MnO}}_4}^{2-}$respectively are $4.90\mathrm{BM},0\mathrm{BM}\mathrm{and}1.73\mathrm{BM}$.