The spin only magnetic moment in strong ligand field of $d^{6}$ ion of octahedral complex is:

Question

Accepted Answer

The question asks for the spin-only magnetic moment of a $d^6$ ion in an octahedral complex with a strong ligand field. Let's analyze why Option 2, which states the magnetic moment is 0, is correct.

### Explanation for Option 2 (Correct Answer):

1. **Strong Ligand Field and Low Spin Configuration:**
   - In an octahedral complex with a strong ligand field, the ligands cause a large splitting of the d-orbitals into two sets: the lower energy t2g orbitals and the higher energy eg orbitals.
   - For a $d^6$ ion, in a strong ligand field, the electrons will pair up in the lower energy t2g orbitals to minimize energy, resulting in a low spin configuration.
   - The electron configuration in this case would be t2g^6 eg^0, meaning all six electrons are paired in the t2g orbitals.

2. **Spin-Only Magnetic Moment:**
   - The spin-only magnetic moment is calculated using the formula: $\mu = \sqrt{n(n+2)}$, where $n$ is the number of unpaired electrons.
   - In the low spin configuration of a $d^6$ ion in a strong ligand field, there are 0 unpaired electrons.
   - Substituting $n = 0$ into the formula gives $\mu = \sqrt{0(0+2)} = 0$.

Therefore, the spin-only magnetic moment is 0, which supports Option 2 as the correct answer.

### Explanation for Why Other Options are Incorrect:

1. **Option 1 (4.9 D):**
   - This value would correspond to a situation with 4 unpaired electrons, calculated as $\mu = \sqrt{4(4+2)} = \sqrt{24} \approx 4.9$.
   - A $d^6$ ion in a strong ligand field does not have 4 unpaired electrons, as explained above.

2. **Option 3 (1.73 D):**
   - This value corresponds to 1 unpaired electron, calculated as $\mu = \sqrt{1(1+2)} = \sqrt{3} \approx 1.73$.
   - In a strong ligand field, a $d^6$ ion has no unpaired electrons, making this option incorrect.

3. **Option 4 (2.83 D):**
   - This value corresponds to 2 unpaired electrons, calculated as $\mu = \sqrt{2(2+2)} = \sqrt{8} \approx 2.83$.
   - Again, in a strong ligand field, a $d^6$ ion has all electrons paired, so this option is incorrect.

In conclusion, the correct answer is Option 2, as the strong ligand field causes all electrons to pair, resulting in a spin-only magnetic moment of 0.