Base hydrolysis of mer-trispicolinatoruthenium(III): kinetics and mechanism

The mer-[Ru(pic)3] isomer, where pic is 2-pyridinecarboxylic acid, undergoes base hydrolysis at pH > 12. The reaction was monitored spectrophotometrically within the UV–Vis spectral range. The product of the reaction, the [Ru(pic)2(OH)2]− ion, is formed via a consecutive two-stage process. The chelate ring opening is proceeded by the nucleophilic attack of OH− ion at the carbon atom of the carboxylic group and the deprotonation of the attached hydroxo group. In the second stage, the fast deprotonation of the coordinated OH− ligand leads to liberation of the monodentato bonded picolinate. The dependence of the observed pseudo-first-order rate constant on [OH−] is given by $$ k_{{{obs}1}} = {{k₊ k₁ [{OH}⁻ ] + k₊ k₂ K₁ [{OH}⁻ ]² }}{{k₋ + k₁ + ( {k₊ + k₂ K₁ } )[{OH}⁻ ] + k{}₊K₁ [{OH}⁻ ]² }} $$ and $$ ( {k_{{{obs}2}} = {{kca + kcb K₂ [{OH}⁻ ]}}{{1 + K₂ [{OH}⁻ ]}}} ) $$ for the first and the second stage, respectively, where k 1, k 2, k -, k ca , k cb are the first-order rate constants and k + is the second-order one, K 1 and K 2 are the protolytic equilibria constants.