The second-order rate constants for the mercury(II)-induced aquation of [Rh(NH3)5X]2+ cations (where X = Cl or Br) have been measured in aqueous solution over a range of temperatures. The results are interpreted in terms of the formation of an activated complex RhXHg, which yields the aquopentamminerhodium(III) product by a unimolecular aquation with HgX+ as the leaving group. The effect of changing X from Cl to Br on the activation enthapy of the reaction is discussed, and comparison is made with data on the corresponding spontaneous aquation.
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