For the octahedral complexes of `Fe^(3+)` in `SCN^(-)` (thiocyanato-S) and in `CN^(-)` ligand environments, the difference between the spin-only magnetic moments in B.M. (when approximated to the nearest integar)

Answer

4

Explanation

In the first complex, `[Fe(SCN)_6]^(3-)` the ligand `SCN^(-)` (S is the donor atom) is a weak field ligand and produces a weak ligand field. It forms a high spin complex. Ln `Fe^(3+)` ion (`3d^5` configration), and all the electrons remain unpaired. Spin only magnetic moment `(mu_s) = sqrt(n ( n + 2)) BM`
` = sqrt(5 (5 + 2)) BM = sqrt35BM = 5.916 BM.` In the other complex `[Fe(CN)_6]^(3-)` the ligand `CN^(-)` (C is the donor atom) si a strong field ligand and produces a strong ligand filed. It forms a low spin complex in which four 3d electrons pair up.
Spin only magnetic moment ` (mu_s) = sqrt(n (n +2)) = sqrt(1(1 + 2)) = 3 BM = 1.732 BM`
Difference in the magnetic moments `= 5.916 - 1.732 = 4.184 ~~ 4B.M.`