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

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: H= 41 ± 2 kJ mol–1, S=–114 ± 7 J K–1 mol–1 and =–13.6 ± 0.3 cm3 mol–1 for [Ru(NH3)6]3+; H= 34.4 ± 1.0 kJ mol–1, S=–132 ± 3 J K–1 mol–1 and V=–18.0 ± 0.5 cm3 mol–1 for [Ru(NH3)5Cl]2+; and H= 31.0 ± 0.7 kJ mol–1 and S=–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.