Electrochemical and chemical aspects of ruthenium(II) and (III) ammines in basic solution: The role of the ruthenium(IV) species

• The reaction of [RuIII(NH3)6]3+ in basic medium yields cis-[RuIII(NH3)4(H2O)2]3+.
• In basic solutions, RuIV was observed after RuIII disproportionation reaction.
• The pKa of complex cis-[RuIII(NH3)4(H2O)2]3+ was calculated as 4.1.
• By DFT, the geometry of the pentacoordinated intermediate is square pyramidal.

The dissolution of the complexes [RuIII(NH3)6]3+ and [RuIII(NH3)5(H2O)]3+ in alkaline media (CNaOH ⩾ 10−3 M) yield the complex cis-[RuIII(NH3)4(OH)2]+ as probed by UV–Vis, CV and EPR. The analysis of the voltammetric spectra of [RuIII(NH3)6]3+ and [RuIII(NH3)5(H2O)]3+ solutions, under the same experimental condition (CNaOH ⩾ 10−3 M), leads to the same conclusion. Electrochemical data also suggest that the oxidation of Ru(III) in alkaline media occurs following the deprotonation of one equatorial NH3 ligand and it produces unstable Ru(IV) species. For the hexaammine complex ion the formation of an amide complex of Ru(IV) is proposed. According to experimental data the stabilization of Ru(IV) through 3pπ → 4dπ bonding of the amido group is quite relevant. The electrochemical parameters α and ks for the oxidation of Ru(III) to Ru(IV) in the penta and hexaammine systems are the same within the limits of experimental error (α = 0.50 ± 0.05; ks = 6.8 ± 0.5 × 10−8 cm s−1; 25.0 ± 0.1 °C; μ = 0.10 M, NaCF3COO, pH 10–11).

As observed by EPR, UV–Vis, and CV data, [RuIII(NH3)6]3+ and [RuIII(NH3)5(H2O)]3+, on standing in alkaline media (CNaOH ⩾ 0.1 M), undergoes a series of reactions yielding as the main product cis-[RuIII(NH3)4(H2O)2]3+. Besides the relevance on its own right this reaction can be explored as a possible new and simple alternative route for the synthesis of cis-Ru(III) complexes.Figure optionsDownload as PowerPoint slide