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

The Correct Option is A

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).