What can be inferred from the magnetic moment of the following complex species?


Example.



Magnetic moment



[K4 [Mn(CN)6]



2.2



[Fe(H2O)6]2+.



5.3



K2 [MnCl4]



5.9


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


μ =√n(n+2) BM


When n= 1



⇒ μ = √1(1+2)


⇒ u= √3


⇒ u=1.73 BM



When n=2



⇒ μ = √2(2+2)


⇒ μ = √8


⇒ μ = 2.83 BM



When n= 3



⇒ μ = √3(3+2)


⇒ μ = 15


⇒ μ = 3.87 BM



When n = 4



⇒ μ = √ 4(4+2)


⇒ μ = √24


⇒ μ = 4.899 BM



When n= 5



⇒ μ = √5(5+2)


⇒ μ = √35


⇒ μ = 5.92 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.


Theatomic 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.


Theatomic 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.



Theatomic 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


Note: 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.


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