Question

Which one of the following complexes will have $\Delta_{0}=0$ and $\mu=5.96$ B.M.?

• A. $\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4}$
• B. $\left[\mathrm{CO}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}$
• C. $\left[\mathrm{FeF}_{6}\right]^{4}$
• D. $\left[\mathrm{Mn}(\mathrm{SCN})_{6}\right]^{4}$

Hint:

The magnetic moment and crystal field stabilization energy depend on the ligand field strength.

Answer 1

The Correct Option is D

1. $\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4}$: - $\mathrm{Fe}^{2+} \Rightarrow 3 \mathrm{~d}^{6} 4 \mathrm{~s}^{0}$ - $\mathrm{CN}^{-}$ is a strong field ligand. - $\mu = 0$ 
2. $\left[\mathrm{CO}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}$: - $\mathrm{Co}^{3+} \Rightarrow 3 \mathrm{~d}^{6} 4 \mathrm{~s}^{0}$ - $\mathrm{NH}_{3}$ is a strong field ligand. - $\mu = 0$ 
3. $\left[\mathrm{FeF}_{6}\right]^{4}$: - $\mathrm{Fe}^{2+} \Rightarrow 3 \mathrm{~d}^{6} 4 \mathrm{~s}^{0}$ - $\mathrm{F}^{-}$ is a weak field ligand. - $\mu = 0$ 
4. $\left[\mathrm{Mn}(\mathrm{SCN})_{6}\right]^{4}$: - $\mathrm{Mn}^{2+} \Rightarrow 3 \mathrm{~d}^{5} 4 \mathrm{~s}^{0}$ - $\mathrm{SCN}^{-}$ is a weak field ligand. - $\mu = \sqrt{35} \mathrm{~BM} = 5.96 \mathrm{~BM}$ - $\Delta_{0} = 0$ 
Therefore, the correct answer is (4) $\left[\mathrm{Mn}(\mathrm{SCN})_{6}\right]^{4}$.