The unusually fast reactions between ruthenium(iii)-ammine complexes and NO revisited

Abstract

The kinetics of the unusually fast reactions between [Ru^III(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 (k_NH3 = 0.30 ± 0.01 M⁻¹ s⁻¹, k_Cl⁻ = 0.75 ± 0.03 M⁻¹ s⁻¹, k_H2O = 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 [Ru^II(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.