Explain why [Fe(H₂O)₆]³⁺ has high magnetic moment value of 5.92 BM whereas magnetic moment of [Fe(CN)₆]^3– has value of only 1.74 BM.

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

Oxidation number of Fe is +3

The electrons remain unpaired because H₂O is weak field ligand. Total no. of the unpaired electron, n = 5.

We know,

Spin only magnetic moment is given by:

μ = [n(n + 2)]^1/2

μ = [5×7]^1/2 [As, unpaired electrons are 5]

μ = 5.916BM

Hybridization is sp³d².

In case of [Fe(CN)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

Oxidation number of Fe is +3

Hybridization is d²sp³

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

Thus, total no. of unpaired electrons = 1

We know,

Spin only magnetic moment is given by:

μ = [n(n + 2)]^1/2

μ = [1×3]^1/2 [As, unpaired electrons are 1]

μ = 1.732BM

Therefore, we can see that 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.