Question

What can be inferred from the magnetic moment values of the following complex species?
Example Magnetic Moment (BM)
K4[Mn(CN)6] 2.2
[Fe(H2O)6]2+ 5.3
K2[MnCl4] 5.9

Answer 1

Magnetic moment ($\mu$) is given as $\mu$=$\sqrt{n(n+2)}$
For value n = 1,$\mu$=$\sqrt{1(1+2)}$= $\sqrt 3$=1.732
For value n = 2, $\mu$=$\sqrt{2(2+2)}=\sqrt 8$=2.83
For value n = 3, $\mu$=$\sqrt{3(3+2)}=\sqrt {15}$=3.87
For value n = 4, $\mu$=$\sqrt{4(4+2)}=\sqrt {24}$=4.899
For value n = 5, $\mu$=√$\sqrt{5(5+2)}=\sqrt {35}$=5.92

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(i) K4[Mn(CN)6]
For in transition metals, the magnetic moment is calculated from the spin-only formula. Therefore, $\sqrt{n(n+2)}$=2.2
We can see from the above calculation that the given value is closest to n=1 . Also, in this complex, Mn is in the +2 oxidation state. This means that Mn has 5 electrons in the d-orbital.
Hence, we can say that CN- is a strong field ligand that causes the pairing of electrons.

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(ii) [Fe(H2O)6]2+
$\sqrt{n(n+2)}$=5.3
We can see from the above calculation that the given value is closest to n=4. Also, in this complex, Fe is in the +2 oxidation state. This means that Fe has 6 electrons in the d-orbital.
Hence, we can say that H2O is a weak field ligand and does not cause the pairing of electrons.

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(iii) K2[MnCl4]
$\sqrt{n(n+2)}$=5.9
We can see from the above calculation that the given value is closest to n=5. Also, in this complex, Mn is in the +2 oxidation state. This means that Mn has 5 electrons in the d-orbital.
Hence, we can say that Cl- is a weak field ligand and does not cause the pairing of electrons.