Q39 / 155 · WBJEE 2024 · Chemistry

Which of the following ion/ions is/are diamagnetic?

  • A
    $\left[\mathrm{CoF}_6\right]^{3-}$
  • Correct answer
    B
    $\left[\mathrm{Co}\left(\mathrm{NH}_3\right]_6\right]^{3+}$
  • C
    $\left[\mathrm{Fe}\left(\mathrm{OH}_2\right)_6\right]^{2+}$
  • Correct answer
    D
    $\left[\mathrm{Fe}(\mathrm{CN})_6\right]^4$

Solution

Hint: $\mathrm{Co}^{3+}$ is a $\mathrm{d}^6$ ion. With strong field ligand like $\mathrm{NH}_3,\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_6\right]^{3+}$ has no unpaired electron. For $\mathrm{Co}^{3+}, \mathrm{F}^{-}$is a weak field ligand. $\mathrm{Fe}^{2+}$ is a $\mathrm{d}^6$ ion. With strong field ligand $\mathrm{CN}^{-},\left[\mathrm{Fe}(\mathrm{CN})_6\right]^{4-}$ has no unpaired electron. $\mathrm{For}_{\mathrm{Fe}^{2+}}, \mathrm{H}_2 \mathrm{O}$ is a weak field ligand.

Hence, the correct answer is options: 'B', 'D'.

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