The magnetic moment of $[Fe(H_2O)_6]^{3+}$ is $5.92 \ BM$,whereas the magnetic moment of $[Fe(CN)_6]^{3-}$ is $1.74 \ BM$. Explain why.

(N/A) In $[Fe(CN)_6]^{3-}$,$Fe^{3+}$ has a $3d^5$ configuration. $CN^-$ is a strong field ligand,which causes the pairing of electrons in the $3d$ orbitals,resulting in $d^2sp^3$ hybridization and one unpaired electron $(n=1)$. The magnetic moment $\mu = \sqrt{n(n+2)} = \sqrt{1(1+2)} = \sqrt{3} \approx 1.732 \ BM$.
In $[Fe(H_2O)_6]^{3+}$,$Fe^{3+}$ has a $3d^5$ configuration. $H_2O$ is a weak field ligand,which does not cause pairing of electrons in the $3d$ orbitals,resulting in $sp^3d^2$ hybridization and five unpaired electrons $(n=5)$. The magnetic moment $\mu = \sqrt{5(5+2)} = \sqrt{35} \approx 5.916 \ BM$.