Acid Ionization Constant and Base-Catalyzed Hydrolysis of Ruthenium(III) Hexaammine Ion

The optical spectrum of basic solutions of Ru(NH3)63+ was analyzed considering the presence of both the hydroxide ion-pair and the deprotonated species Ru(NH3)5NH22+. The assumptions that the absorption peak at 402 nm is due to the deprotonated species and that its extinction coefficient is not temperature dependent enabled us to determine the formation constants of the two species separately. At an ionic strength of 0.1 M and a temperature of 25 °C, the formation constant of the ion pair is K1=5.4 ± 0.8 M-1 with ΔH1°=3.7 ± 0.3 kcal mol-1 and ΔS1°=18.6 ± 0.9 eu, while the formation constant of the deprotonated species from the ion pair is K2=1.8 ± 0.2 with ΔH2°=6.1 ± 0.2 kcal mol-1 and ΔS1°=21.6 ± 0.8 eu, and the Ru(NH3)63+ ion has an apparent acid dissociation constant with a pK of 13.1 ± 0.3. The base-catalyzed aquation reaction of Ru(NH3)63+ was found to proceed through two parallel reaction paths which could be described as a pH-independent and a base-catalyzed aquation reaction of the Ru(NH3)5NH22+ ion. The spectrum of Ru(NH3)5NH22+ was assigned to a ligand-to-metal charge-transfer transition and the optical electronegativity of the NH2- group was estimated as χopt=2.8.