Question

(A) \([\mathrm{MnBr_4}]^{2-}\)
(B) \([\mathrm{Cu(H_2O)_6}]^{2+}\)
(C) \([\mathrm{Ni(CN)_4}]^{2-}\)
(D) \([\mathrm{Ni(H_2O)_6}]^{2+}\)
Select correct order of spin-only magnetic moment among above complexes.

• A. A>D>B>C
• B. D>A>C>B
• C. D>B>A>C
• D. A>B>D>C

Hint:

Spin-only magnetic moment depends {only on unpaired electrons}. Strong-field ligands like \(\mathrm{CN^-}\) often cause pairing and reduce magnetism.

Answer 1

The Correct Option is A

To determine the correct order of spin-only magnetic moment for the given complexes, we need to calculate the number of unpaired electrons in each complex as the spin-only magnetic moment is directly related to unpaired electrons. The formula for spin-only magnetic moment \(\mu_{\text{spin only}}\) is given by:

\(\mu_{\text{spin only}} = \sqrt{n(n+2)}\mu_B\), where \(n\) is the number of unpaired electrons and \(\mu_B\) is the Bohr magneton.

1. Complex (A) \([\mathrm{MnBr_4}]^{2-}\):
   • Manganese is in a +2 oxidation state (Mn²⁺), with configuration \([\text{Ar}] 3d^5 4s^0\).
   • Br^- is a weak field ligand and does not cause pairing of electrons.
   • Number of unpaired electrons = 5.
   • Magnetic moment = \(\mu_{\text{spin only}} = \sqrt{5(5+2)} = \sqrt{35} \approx 5.92 \mu_B\).
2. Complex (B) \([\mathrm{Cu(H_2O)_6}]^{2+}\):
   • Copper is in a +2 oxidation state (Cu²⁺), with configuration \([\text{Ar}] 3d^9 4s^0\).
   • Water is a weak field ligand.
   • Number of unpaired electrons = 1 (in the 3d orbital).
   • Magnetic moment = \(\mu_{\text{spin only}} = \sqrt{1(1+2)} = \sqrt{3} \approx 1.73 \mu_B\).
3. Complex (C) \([\mathrm{Ni(CN)_4}]^{2-}\):
   • Nickel is in a +2 oxidation state (Ni²⁺), with configuration \([\text{Ar}] 3d^8 4s^0\).
   • CN^- is a strong field ligand and causes pairing of electrons.
   • Number of unpaired electrons = 0.
   • Magnetic moment = \(\mu_{\text{spin only}} = 0 \mu_B\).
4. Complex (D) \([\mathrm{Ni(H_2O)_6}]^{2+}\):
   • Nickel is in a +2 oxidation state (Ni²⁺), with configuration \([\text{Ar}] 3d^8 4s^0\). 
   • Water is a weak field ligand and does not cause pairing.
   • Number of unpaired electrons = 2.
   • Magnetic moment = \(\mu_{\text{spin only}} = \sqrt{2(2+2)} = \sqrt{8} \approx 2.83 \mu_B\).

Based on these calculations, the order of spin-only magnetic moment is:

(A) > (D) > (B) > (C)

Therefore, the correct answer is A>D>B>C.