Explain why `[Fe(H_(2)O)_(6)]^(3+)` has magnetic moment value of `5.92` BM whereas `[Fe(CN)_(6)]^(3-)` has a value of only `1.74` BM ?

Explanation

Both these complexes are octahedral in nature but have different states of hybridisation. `[Fe(CN)_(6)]^(3-)` involves `d^(2)sp^(3)` hybridisation with one unpaired electron (inner orbital complex). Its magnetic moment is low `(1.74)` BM.
The other complex `[Fe(H_(2)O)]^(3+)` involves `sp^(3)d^(2)` hybridisation with five unpaired electrons. Its magnetic moment is quite high `(5.92)` BM. The difference in magnetic moment and state of hybridisation is because of nature of ligands. Whereas `CN^(-)` ion is a strong field ligand, `H_(2)O` represents a weak field ligand.