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

In the free state `Fe^(3+)` ion has the electronic configuration `3d^(5)(=d^(1)d^(1)d^(1)d^(1))`. In the presence of weak ligand `H_(2)O`, these electrons do not pair up. So `Fe^(3+)` undergoes `sp^(3)d^(2)` hybridisation forming outer orbital octahedral complex containing 5 unpaired electrons. Thus `mu=sqrt(n(n+2))=sqrt(5(5+2))=5.92` BM. But in the presence of strong ligand `(CN^(-))`, the d-electrons of `Fe^(+)` pair up to give the configuration `d^(2)d^(2)d^(1)d^(0)d^(0)`. So `Fe^(3+)` undergoes `d^(2)sp^(3)` hybridisation forming inner orbital octahedral complex containing one unpaired electron `(n=1)`. So `mu=sqrt(1(1+2))-1.73` BM.