Question

Which one of the following complexes will exhibit the least paramagnetic behaviour ? [Atomic number, Cr = 24, Mn = 25, Fe = 26, Co = 27]

• A. [Co(H2O)6]+2
• B. [Fe(H2O)6]+2
• C. [Mn(H2O)6]+2
• D. [Cr(H2O)6]+2

Answer 1

Answer: (A)

[Co(H2O)6]+2

Answer 2

The paramagnetic behaviour depends on the number of unpaired electrons. Using the electronic configurations:

$Complex$	$\text{Number of unpaired electrons}$	$\mu = \sqrt{n(n+2)}$ B.M.
$[\text{Co(H}_2\text{O)}_6]^{2+}$	3	3.87
$[\text{Fe(H}_2\text{O)}_6]^{2+}$	4	4.89
$[\text{Mn(H}_2\text{O)}_6]^{2+}$	5	5.92
$[\text{Cr(H}_2\text{O)}_6]^{2+}$	4	4.89

The least paramagnetic complex is $[\text{Co(H}_2\text{O)}_6]^{2+}$, as it has the fewest unpaired electrons (3).