#### Question

[Fe(H_{2}O)_{6}]^{3+} is strongly paramagnetic whereas [Fe(CN)_{6}]^{3−} is weakly paramagnetic. Explain.

#### Solution

In both [Fe(H_2O)_6]^(3+) and [Fe(CN)_6]^(3-) Fe exists in the +3 oxidation state i.e., in *d*^{5} configuration.

Since CN^{−} is a strong field ligand, it causes the pairing of unpaired electrons. Therefore, there is only one unpaired electron left in the *d*-orbital.

Therefore,

`mu = sqrt(n(n+2))`

`=sqrt(1(1+2))`

`=sqrt3`

=1.732 BM

On the other hand, H_{2}O is a weak field ligand. Therefore, it cannot cause the pairing of electrons. This means that the number of unpaired electrons is 5.

Therefore,

`mu = sqrt(n(n+2))`

`=sqrt(5(5+2))`

=sqrt35

=6 BM

Thus it is evident that `[Fe(H_2O)_6]^(3+)` is strongly paramagnetic, while `[Fe(CN)_6]^(3-)`is weakly paramagnetic