Kinetics and mechanism of the reduction of thiocyanato-, azido-, and isothiocyanato-penta-amminecobalt(III) by hexa-ammineruthenium(II), tris(ethylenediamine)ruthenium(II), and tris(2,2′-bipyridyl)chromium(II) in aqueous solutions

Abstract

The kinetics of reduction of [CO(NH₃)₅X]²⁺(X = SCN, N₃, or NCS) by [Ru(NH₃)₆]²⁺ and [Ru(en)₃]²⁺(en = ethylenediamine) in aqueous solutions have been investigated. All the reactions follow second-order rate equations, and the kinetic parameters obtained at 25 °C, I= 0.20 mol dm^–3, and [H⁺]= 0.10 mol dm^–3 are as follows: [Ru(NH₃)₆]²⁺, k_SCN= 379 ± 7, k_N3= 1.82 ± 0.10, k_NCS= 0.74 ± 0.06 dm³ mol^–1 s^–1, ΔH_SCN^‡= 6.4 ± 0.2 kcal mol^–1, ΔS_SCN^‡=–(25.3 ± 1.0) cat K^–1 mol^–1, ΔH_N3^‡= 9.4 ± 0.8 kcal mol^–1, ΔS_N3^‡=–(25.9 ± 2.6) cal K^–1 mol^–1, ΔH_NCS^‡= 14.9 ± 0.7 kcal mol^–1, ΔS_NCS^‡=–(9.5 ± 1.8) cal K^–1 mol^–1; [Ru(en)₃]²⁺, k_SCN= 45.3 ±4.0, k_N3= 0.26 ± 0.01, k_NCS= 0.15 ± 0.02 dm³ mol^–1 s^–1; [Cr(bipy)₃]²⁺(bipy = 2,2′-bipyridyl), k_SCN 7.0 × 10⁶ dm³ mol^–1 s^–1. The small k_N3: k_NCS ratios for these reactions confirm that they proceed by the outer-sphere mechanism. The k_SCN: k_N3 ratio is generally higher for reactions of the outer-sphere mechanism than for reactions of the inner-sphere type in which the bridging ligands are only [SCN]^–, [N₃]^–, and [NCS]^–. This mechanism-dependent reactivity pattern is attributed to the fact that in outer-sphere reactions the sole determinant of the reactivity pattern is ΔG^⊖, while the crucial factor in inner-sphere reactions is the relative stability of the precursor complexes. Additional confirmatory evidence for this conclusion stems from the excellent correlation obtained when Marcus' theory is employed to compare the rates of reduction of some penta-ammineanionocobalt(III) complexes by [Cr(bipy)₃]²⁺ and [Ru(NH₃)₆]²⁺.