Question
$\left[ Fe \left( H _2 O \right)_6\right]^{3+}$ is strongly paramagnetic whereas $\left[ Fe ( CN )_6\right]^{3-}$ is weakly paramagnetic. Explain.

Answer

In both the complexes, Fe is in +3 oxidation state with the configuration $3 d^5 . CN ^{-}$is a strong field ligand. In its presence, 3 d electrons pair up leaving only one unpaired electron. The hybridisation is $d ^2 sp ^3$ forming inner orbital complex. $H _2 O$ is a weak ligand. In its presence $3 d$ electrons do not pair up. The hybridisation is $sp ^3 d^2$ forming an outer orbital complex containing five unpaired electrons, hence it is strongly paramagnetic.

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