Q32 / 160 · TS EAMCET 2020 (14 Sep Shift 1) · Chemistry

Among the following $\mathrm{Cr}(\mathrm{III})$ complexes, which one will have the highest octahedral crystal field splitting?

  • A
    $\left[\mathrm{CrF}_6\right]^{3-}$
  • B
    $\left[\mathrm{Cr}\left(\mathrm{H}_2 \mathrm{O}\right)_6\right]^{3+}$
  • C
    $\left[\mathrm{Cr}\left(\mathrm{NH}_3\right)_6\right]^{3+}$
  • Correct answer
    D
    $\left[\mathrm{Cr}(\mathrm{CN})_6\right]^{3-}$

Solution

A strong field ligand will enable higher crystal field splitting. It can be predicted from the relative position of a ligand in spectrochemical series. In the given octahedral complexes of $\mathrm{Cr}(\mathrm{II})$ ion, the ligands are $\mathrm{F}^{-}, \mathrm{H}_2 \mathrm{O}, \mathrm{NH}_3$ and $\mathrm{CN}^{-}$and their increasing order of field strength in spectrochemical series is $\mathrm{F}^{-} < \mathrm{H}_2 \mathrm{O} < \mathrm{NH}_3 < \mathrm{CN}^{-} \text {. }$ So, highest octahedral crystal field splitting is present in $\left[\mathrm{Cr}(\mathrm{CN})_6\right]^{3-}$.

Hence, the correct answer is option: 'D'.

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