Solve the following


[Fe(H2O)6]3+ is strongly paramagnetic whereas [Fe(CN)6]3− is weakly paramagnetic. Explain.


In both $\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}$ and $\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-}, \mathrm{Fe}$ 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.





$=1.732 \mathrm{BM}$

On the other hand, H2O is a weak field ligand. Therefore, it cannot cause the pairing of electrons. This means that the number of unpaired electrons is 5.





$\simeq 6 \mathrm{BM}$

Thus, it is evident that $\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}$ is strongly paramagnetic, while $\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-}$ is weakly paramagnetic.



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