Coordination Compounds 2 Question 60
####62. For the octahedral complexes of $Fe^{3+}$ in $SCN^{-}$ (thiocyanato-S) and in $CN^{-}$ligand environments, the difference between the spin only magnetic moments in Bohr magnetons (when approximated to the nearest integer) is [atomic number of $Fe=26$ ]
(2015 Adv.)
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Solution:
- When $S$ is donor atom of $SCN^{-}$, it produces weak ligand field and forms high spin complex as
$\left[Fe(SCN) _6\right]^{3-}: Fe^{3+}\left(3 d^{5}\right)=$
Spin only magnetic moment $\left(\mu _s\right)=\sqrt{5(5+2)} BM=\sqrt{35} BM$
In case of $CN^{-}$ligand, carbon is the donor atom, it produces strong ligand field and forms low spin complex as
$\left[Fe(CN) _6\right]^{3-}: Fe^{3+}\left(3 d^{5}\right)$
Spin only magnetic moment $\left(\mu _s\right)=\sqrt{1(1+2)} BM=\sqrt{3} BM$
Hence, difference in spin only magnetic moment
$$ =\sqrt{35}-\sqrt{3} \approx 4 BM $$
