In [Fe(H₂O)₆]³⁺

Electronic configuration of Fe is: [Ar]3d⁶4s²

[Ar] = 1s²2s²2p⁶3s²3p⁶

Electronic configuration of Fe⁺³ = [Ar]3d⁵

Outer electronic configuration of Fe⁺³ = 3d⁵

Overall charge balance:

X + 6(0) = 3

X = + 3

In [Fe(CN)₆]^3-

Overall charge balance:

X + 6(-1) = -3

X = + 3

In both the compounds Fe is in + 3 oxidation state.

In case of [Fe(H₂O)₆]³⁺

H₂O is weak field ligand so it does not pair the unpaired electron. Total no. of the unpaired electron, n = 5.

Spin only magnetic moment is given by:

μ = [n(n + 2)]^1/2

μ = [5×7]^1/2

μ = 5.916BM

In case of [Fe(CN)₆]^3-

CN^- is a strong field ligand so it pairs up the electron.

Total no. of unpaired electrons = 1

Spin only magnetic moment is given by:

μ = [n(n + 2)]^1/2

μ = [1×3]^1/2

μ = 1.732BM

as we can see spin only magnetic moment of [Fe(H₂O)₆]³⁺ is more than [Fe(CN)₆]^3- .

so, [Fe(H₂O)₆]³⁺ is strongly paramagnetic whereas [Fe(CN)₆]³ weakly paramagnetic.

Fe in ground state

Fe in + 3 oxidation state

Electrons from 5 CN^- ligands