`[Fe(H_2O)_6]^(3+)` is strongly paramagnetic whereas `[Fe(CN)_6]^3-` is weakly paramagnetic. Explain.

Explanation

In both the complexes, Fe is in +3 oxidation state with the ground state outer electronic configuration : `3d^6`. In its presence of strong ligand `CN^(-)`, 3d-electrons pair up leaving only one unpaired electron. The hybridisation is `d^2 sp^3` forming inner orbital complex. On the other hand `H_2O` is a weak ligand. In its presence, 3d 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.