Question

Among the four complex ions given, identify the pair of ions with same spin only magnetic moment value:

• A. II, III only
• B. II, IV only
• C. I, IV only
• D. I, III only

Hint:

Two complexes have the same spin-only magnetic moment if they contain the same number of unpaired electrons.

Answer 1

The Correct Option is B

Concept: Spin-only magnetic moment is \[ \mu=\sqrt{n(n+2)} \] where \(n\) is the number of unpaired electrons.

Step 1: Find unpaired electrons in each complex.
\[ [Ni(NH_3)_6]^{2+} \] \[ Ni^{2+}=3d^8 \] Unpaired electrons \(=2\) \[ [MnCl_6]^{3-} \] \[ Mn^{3+}=3d^4 \] Weak field ligand \(\Rightarrow\) high spin \[ n=4 \] \[ [FeF_6]^{3-} \] \[ Fe^{3+}=3d^5 \] High spin \[ n=5 \] \[ [CoF_6]^{3-} \] \[ Co^{3+}=3d^6 \] High spin \[ n=4 \]

Step 2: Compare magnetic moments.
Complex II: \[ \mu=\sqrt{4(4+2)} =\sqrt{24} \] Complex IV: \[ \mu=\sqrt{4(4+2)} =\sqrt{24} \] Hence both have identical spin-only magnetic moment. \[ \boxed{\text{II and IV}} \] Therefore, \[ \boxed{\text{Option (B)}} \]