Question

For the octahedral complex of `Fe^(+)` in `SCN^(-)` (thiocyanato-S) and in `CN^(-)` ligand environments, the difference between the spin - only magnetic moments in Bohr magneton (when approximated to the nerest integer) is `[At. No . of Fe = 26]`
A. 3
B. 6
C. 4
D. 5

Answer 1

Correct Answer - C
We have complex:
`[Fe(SCN)_(6)]^(3-)` and `[Fe(CN)_(6)]^(3-)`
In both the complexes, the oxidation number of Fe is `+3` which has the configuration `[Ar]3d^(5)` . Since `SCN^(-)` is a weak field ligand while `CN^(-)` is a strong field ligand, the pairing will take place only in cyanide complex.
`[Fe(SCN)_(6)]^(3-)`

`mu = sqrt(n(n +2)) = sqrt(5(5+2)) = sqrt(35) = 5.91` BM
`[Fe(CN)_(6)]^(3-)`

`mu = sqrt(n(n+)) = sqrt(1(1+2)) = sqrt(3) = 1.73` BM
Difference spin only magnetic moment `=5.91 - 1.73 = 4.18~~4`