The unusually fast reactions between ruthenium(iii)-ammine complexes and NO revisitedElectronic supplementary information (ESI) available: UV-Vis spectrum of [Ru(NH3)5NO]3+ and titration curve for the acid dissociation of [Ru(NH3)5(H2O)]3+. See http://www.rsc.org/suppdata/dt/b2/b210256k/

Abstract

The kinetics of the unusually fast reactions between [RuIII(NH3)5X](3−n)+ (Xn− = Cl−, NH3, H2O) and NO were reinvestigated in acidic aqueous solution in order to clarify the underlying reaction mechanism. The measured second-order rate constants (kNH3 = 0.30 ± 0.01 M−1 s−1, kCl− = 0.75 ± 0.03 M−1 s−1, kH2O = 55.6 ± 3.2 M−1 s−1 at 26 °C) are in good agreement with literature data for X = ammonia and halide. The activation parameters determined for the reactions are: ΔH≠ = 41 ± 2 kJ mol−1, ΔS≠ = −114 ± 7 J K−1 mol−1 and ΔV≠ = −13.6 ± 0.3 cm3 mol−1 for [Ru(NH3)6]3+; ΔH≠ = 34.4 ± 1.0 kJ mol−1, ΔS≠ = −132 ± 3 J K−1 mol−1 and ΔV≠ = −18.0 ± 0.5 cm3 mol−1 for [Ru(NH3)5Cl]2+; and ΔH≠ = 31.0 ± 0.7 kJ mol−1 and ΔS≠ = −108 ± 2 J K−1 mol−1 for [Ru(NH3)5(H2O)]3+. Bond formation with the entering nucleophile appears to be substantial in the transition state for the reaction. An associative substitution mechanism coupled to a concerted electron transfer process to produce [RuII(NH3)5(NO+)]3+ is proposed for all three reactions. Possible reasons for the significantly faster reaction observed in case of the aqua complex are discussed.