Consider the two sets of molecules. Set A: $\left[\mathrm{AlF}_{6}\right]^{3-},\left[\mathrm{PF}_{6}\right]^{-},\left[\mathrm{SF}_{6}\right]$ and $\left[\mathrm{SiF}_{6}\right]^{2-}$ Set B: $\left[\mathrm{Ba}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+},\left[\mathrm{Ca}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+},\left[\mathrm{Mg}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}$, $\left[\mathrm{Sr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}$ The slowest ligand exchange rate in Set A and Set B are, respectively
(a)$\left[\mathrm{AlF}_{6}\right]^{3-}$ and $\left[\mathrm{Sr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}$
(c)$\left[\mathrm{SiF}_{6}\right]^{2-}$ and $\left[\mathrm{Ca}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}$
(b)$\left[\mathrm{SF}_{6}\right]$ and $\left[\mathrm{Mg}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}$
(d)$\left[\mathrm{PF}_{6}\right]^{-}$ and $\left[\mathrm{Ca}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}$
Answer
Answer: C ✓ checked by 4AB · confidence high
The source book printed B; on checking, C is correct — see the explanation.
Explanation
Set A: SF₆ is the most inert (neutral, highly charged S(VI), no route for attack). Set B: the smallest, most strongly bound aqua ion Mg²⁺ exchanges water most slowly.