For the octahedral complexes of `Fe^(3+)` is `SCN^(-)` ( thiocyanato-S) and in `CN^(-)` ligand environments , the difference between the spin only magnetic moments in Bohr magnetons (when approximated to the nearest integer ) is [atomic number of Fe = 26]

Explanation

In the first complex, `[Fe(SCN)_6]^(3-)` the ligand `SCN^(-)` (S is the donor atom ) is a weak field ligand and products a weak ligand field. It forms a high spin complex. In `Fe^(3+)` ion (`3d^(5)` configuration) , and all the electrons remain unpaired.

Spin only magnetic moment `(mu_S)` = `sqrt n(n + 2)BM`
=`sqrt 5(5 + 2) BM` = `sqrt 35 BM` = 5.916 BM.
In the other complex `[Fe(CN)_6]^(3-)` the ligand `CN^(-)` ( C is the donor atom) is a strong field ligand and produces a strong ligand field. It forms a low spin complex in which four 3d electrons pair up.

Spin only magnetic moment (`mu_S`) = `sqrt n(n+2)`
`sqrt n(n + 2)` = 3 BM = 1.732 BM
Difference in the magnetic moments
= 5.916 - 1.732
= 4.184 `approx` 4 BM