Base hydrolysis of $\left[\mathrm{CoCl}\left(\mathrm{NH}_{3}\right)_{5}\right]^{2+}$ is an overall second order reaction, whereas that of $[\mathrm{Co}(\mathrm{CN})]^{3-}$ is of first order. The rates depend in both cases solely on the concentrations of the cobalt complex. This may be due to
\[ \begin{array}{l} \text{(A) Presence of ionizable proton in } \left[\mathrm{CoCl}\left(\mathrm{NH}_{3}\right)_{5}\right]^{2+} \text{ but not in } \left[\mathrm{Co}(\mathrm{CN})_{6}\right]^{3-}, \\ \text{(B) } \mathrm{SN}^{1}{ }_{\mathrm{CB}} \text{ mechanism in the case of } \left[\mathrm{CoCl}\left(\mathrm{NH}_{3}\right)_{5}\right]^{2+} \text{ only} \\ \text{(C) } \mathrm{SN}^{1}{ }_{\mathrm{CB}} \text{ mechanism in the case of } \left[\mathrm{Co}(\mathrm{CN})_{6}\right]^{3-}, \text{ only} \\ \text{(D) } \mathrm{SN}^{1}{ }_{\mathrm{CB}} \text{ mechanism in both the complexes} \end{array} \]
Correct explanation(s) is/are
(a)A and B
(b)A and C
(c)B only
(d)A and D
Answer
Answer: A ✓ checked by 4AB · confidence high
Answer: (a) and (b).
Explanation
[CoCl(NH₃)₅]²⁺ has acidic N–H protons (statement A). OH⁻ removes one of them to give the amido conjugate base [CoCl(NH₂)(NH₃)₄]⁺, which loses Cl⁻ in the rate-determining step. This is the SN1CB mechanism (statement B), and it makes the rate depend on [OH⁻] as well, so the reaction is second order overall. [Co(CN)₆]³⁻ has no ionisable proton, so SN1CB is impossible for it and its hydrolysis is first order. Statements A and B are correct, so the answer is (a).