Kinetics and mechanisms of the reduction of a cis-dioxoruthenium(VI) complex by [Ni(tacn)2]2+ and [Fe(H2O)6]2+

The kinetics of the reduction of cis-[Ru^VIL(O)₂]²⁺ (L = N,N,N′,N′-tetramethyl-3,6-dimethyl-3,6-diazaoctane-1,8-diamine) by [Ni(tacn)₂]²⁺ (tacn = 1,4,7-triazacyclononane) and [Fe(H₂O)₆]²⁺ have been studied in aqueous acidic solutions. Both reactions have the following stoichiometry: 2M^II + cis-[Ru^VIL(O)₂]²⁺ + 2H⁺ → 2M^III + cis-[Ru^IVL(O)(OH₂)]²⁺ (M = Ni or Fe). Two distinct steps were observed for both reactions and these are assigned to Ru^VI → Ru^V and Ru^V → Ru^IV. Both steps are first order in [Ru^VI] and [M^II]. For the reduction by [Ni(tacn)₂]²⁺, the activation parameters (I = [H⁺] = 0.1 mol dm^-3) for the first step are ΔH‡ = 13.4 ± 1.0 kJ mol^-1 and ΔS‡ = -111 ± 10 J mol^-1 K^-1; for the second reaction, ΔH‡ = 28.5 ± 1.5 kJ mol^-1 and ΔS‡ = -110 ± 10 J mol^-1 K^-1. The rate constant for the first step is independent of acid concentration, an outer-sphere mechanism is proposed and a self-exchange rate of 2 × 10⁴ dm³ mol^-1 s^-1 for the cis-[Ru^VIL(O)₂]²⁺/cis-[Ru^VL(O) ₂]⁺ couple is estimated using the Marcus cross-relation. The rate constant of the second step increases with [H⁺] and it reaches saturation at high [H⁺]. A mechanism involving a pre-equilibrium protonation of cis-[Ru^VL(O)₂]⁺ followed by outer-sphere electron transfer is proposed. For the reduction by [Fe(H₂O)₆]²⁺, rate constants for both steps are independent of acid concentration in the range of pH = 1-3. The activation parameters (I = 1.0 mol dm^-3, pH = 1.0) for the first step are ΔH‡ = 32.5 ± 1.5 kJ mol^-1 and ΔS‡ = -52.5 ± 7 J mol^-1 K^-1; while for the second step, ΔH‡ = 17.3 ± 1.2 kJ mol^-1 and ΔS‡ = -140 ± 13 J mol^-1 K^-1. An outer-sphere mechanism is proposed for the first step and an inner-sphere mechanism is proposed for the second step.