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).

Answer 2

Step 1: Write down the complexes to compare
The question asks which among the given complexes will show the least paramagnetic behaviour. We are comparing complexes that involve transition metal ions — Cr, Mn, Fe, and Co — each forming hexaaqua complexes such as [Cr(H₂O)₆]²⁺, [Mn(H₂O)₆]²⁺, [Fe(H₂O)₆]²⁺, and [Co(H₂O)₆]²⁺.

Step 2: Recall what “paramagnetic behaviour” means
Paramagnetism arises due to the presence of unpaired electrons in the d-orbitals of the metal ion. Greater the number of unpaired electrons → stronger paramagnetism. Fewer unpaired electrons → weaker (less) paramagnetism.

Step 3: Determine the oxidation states and electronic configurations
For each metal ion:

• Cr²⁺: Atomic number 24 → configuration [Ar]3d⁴ (4 unpaired electrons)
• Mn²⁺: Atomic number 25 → configuration [Ar]3d⁵ (5 unpaired electrons)
• Fe²⁺: Atomic number 26 → configuration [Ar]3d⁶ (4 unpaired electrons)
• Co²⁺: Atomic number 27 → configuration [Ar]3d⁷ (3 unpaired electrons)

Step 4: Compare the number of unpaired electrons
Among these ions in their hexaaqua complexes, the number of unpaired electrons decreases as: \[ \text{Mn}^{2+} (5) > \text{Cr}^{2+} (4) \approx \text{Fe}^{2+} (4) > \text{Co}^{2+} (3) \] Thus, [Co(H₂O)₆]²⁺ will have the fewest unpaired electrons and therefore the least paramagnetic behaviour.

Step 5: Conceptual reasoning
In aqueous solution, water acts as a weak ligand (high-spin complexes). The number of unpaired electrons depends mainly on the electronic configuration of the metal ion, not on pairing effects. Therefore, the comparison above remains valid.

Final answer

[Co(H₂O)₆]²⁺