From Alkali Inhibition to Hydrophobic Acceleration: Cation Environment Effect on [RhCl6]3– Base Hydrolysis

The cationic environment exerts unexpectedly decisive control over the base hydrolysis of [RhCl6]3–, enabling the reaction rate to be tuned across 2 orders of magnitude simply by choosing the countercation. In the dissociative pathway, the reaction is strongly inhibited in alkaline media due to the formation of contact ion pairs between the complex anion and the cation, as quantitatively described by the Pitzer model. In contrast, replacement with tetraalkylammonium (R4N+) cations reverses this trend, accelerating the hydrolysis rate by up to 2 orders of magnitude relative to KOH. This phenomenon arises from the encapsulation of [RhCl6]3– within water-rich hydrophobic pockets of the R4N+ network, which preserves the hydration shell essential for dissociative activation of the Rh–Cl bond. Molecular dynamics simulations reveal that while K+ displaces water from the first coordination sphere, bulky R4N+ cations form solvent-separated ion pairs, leaving the hydration environment intact. The phenomenon is not unique to rhodium: similar cation-driven modulation is observed for [IrCl6]3– hydrolysis and even for the aquation of [RhCl6]3–, pointing to a general mechanistic principle. These findings challenge the view of alkali cations as innocent spectators and offer a new strategy for controlling inert metal complex reactivity through cation engineering.