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

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