Block D and F Elements

8.48 To calculate magnetic moment of the complex species, we use the spin formula:

μ =√n(n+2) BM

1. [K₄ [Mn(CN)₆]

⇒μ = 2.2 BM (given)

We can see from the above calculation that the given value(2.2) is close to n=1. It means that it has only one unpaired electron Also in this complex Mn is in +2 oxidation state,i.e., as Mn²⁺.Thus when CN^- ligands approach Mn²⁺ ion, The electrons in 3d do not pair up.

The atomic number of Manganese (Mn) is Z = 25

The electronic configuration of ₂₅Mn= [Ar] 3d⁵ 4s²

And, the electronic configuration of Mn²⁺=[Ar] 3d⁵

Thus CN^- is a strong ligand.

The hybridization involved is d²sp³ forming inner orbital octahedral complex

2. [Fe(H2O)₆]^2+.

⇒ μ = 5.3 BM (given)

We can see from the above calculation that the given value(5.3) is close to n = 4. It means that it has four unpaired electrons. In this complex, Fe is in +2 state, i.e., as Fe²⁺. This means that the electrons in 3d do not pair up when the ligands, H₂O molecules approach.

The atomic number of Iron(Fe)is Z = 26

The electronic configuration of ₂₆Fe= [Ar] 3d⁶ 4s²

And, the electronic configuration of Fe²⁺=[Ar] 3d⁶

Thus H₂O is a weak ligand. To accommodate the electrons donated by six H₂O molecules, the hybridization will be sp³d². Hence it will be an outer orbital octahedral complex.

3. K₂ [MnCl₄]

⇒μ = 5.9 BM (given)

We can see from the above calculation that the given value(5.9) is close to n=5. It means that it has five unpaired electrons

In this complex, Mn is in +2 state, i.e., as Mn²⁺. Hence, we can say that Cl- is a weak ligand and does not cause the pairing of electrons.

The atomic number of Manganese (Mn) is Z = 25

The electronic configuration of ₂₅Mn= [Ar] 3d⁵ 4s²

And, the electronic configuration of Mn²⁺=[Ar] 3d⁵

Thus, the hybridization involved will be sp³ and the complex will be tetrahedral complex

Being Lewis bases (those who donate electrons) the ligands with less electronegativity will be stronger. Therefore, in general halogen or oxygen donors (F^-, Cl^-,Br^-, H₂O) are weak field ligands and the ones in which carbon or nitrogen atom is the donor (eg-CN^-,CO,NH₃) are strong field ligands.