Q58 · CSIR-NET Chemistry, December 2015

Paper: CSIR-NET December 2015 · Subject: Inorganic Chemistry · Chapter: Coordination Chemistry · Topic: Electron Transfer Redox · Marks: 2 · Difficulty: Medium

Aqueous $\mathrm{Cr}^{2+}$ effects one electron reduction of $\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{Cl}\right]^{2+}$ giving compound Y. Compound Y undergoes rapid hydrolysis. Y is,
(a)$\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5}\right]^{2+}$
(b)$\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5}(\mathrm{OH})\right]^{+}$
(c)$\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4}(\mathrm{OH})_{2}\right]$
(d)$\left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5} \mathrm{Cl}\right]^{2+}$
Answer
Answer: A ✓ checked by 4AB · confidence medium
Explanation
Inner-sphere reduction transfers Cl⁻ to chromium, leaving the labile Co(II) pentaammine (with a water in the sixth site), which rapidly hydrolyses to [Co(H₂O)₆]²⁺.

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