`[Fe(H_(2)O)_(6)]^(3+)` is strongly paramagnetic whereas `[Fe(CN)_(6)]^(3-)` is weakly paramagnetic. Explain.

Explanation

In both the complexes, the metal ion is `Fe^(3+)(3d^(5)4s^(0)" confg.")`. In presence of `CN^(-)` ion (strong ligand), 3d - electrons pair up leaving only one unpaired electron. It is followed by `d^(2)sp^(3)`- hybridisation, which involves 3d, 4s and 4p - orbitals. Finally, `Fe^(3+)` ion combines with six `CN^(-)` ions to form inner orbital complex `[Fe(CN)_(6)]^(3-)`, having an octahedal structure. It contains only one unpaired electron and hence weakly paramagnetic in nature.

In presence of `H_(2)O` (weak ligand), 3d - electrons of `Fe^(3+)` ion do not pair up. Thus `sp^(3)d^(2)` hybridisation occurs, involving `4s, 4p` and 4d 0 orbitals (only two). `Fe^(3+)` ion then combines with six `H_(2)O` molecules to form outer orbital complex `[Fe(H_(2)O)_(6)]^(3+)` having an octahedral structure. It contains five unpaired electrons and hence strongly paramagnetic in nature.