Kinetics and mechanism of the reduction of thiocyanato- and isothiocyanato-penta-amminecobalt(III) ions by europium(II)

Abstract

The kinetics of reduction of [Co(NH₃)₅(SCN)]²⁺ and [Co(NH₃)₅(NCS)]²⁺ by Eu²⁺ have been investigated in acid solution. For both complexes the reactions are first order in concentrations of the cobalt complex and Eu²⁺. At 25 °C, [H⁺]= 0.1M, and I= 1.0M(Li[ClO₄]), k=(3.1 ± 0.1)× 10³ and (4.95 ± 0.13)× 10^–2 l mol^–1 s^–1 for [Co(NH₃)₅(SCN)]²⁺ and [Co(NH₃)₅(NCS)]²⁺ respectively. The corresponding activation parameters are ΔH^‡= 2.7 ± 0.2 kcal mol^–1 and ΔS^‡=–33.4 ± 5.6 cal K^–1 mol^–1, and ΔH^‡= 7.5 ± 0.7 kcal mol^–1 and ΔS^‡=–39.7 ± 2.7 cal K^–1 mol^–1. The reactivity order [Co(NH₃)₅(SCN)]²⁺ > [Co(NH₃)₅(N₃)]²⁺ > [Co(NH₃)₅(NCS)]²⁺(k_SCN : k_N3 : k_NCS= 6.5 × 10⁴ : 3.9 × 10³ : 1) is discussed in terms of an inner-sphere mechanism in which Eu²⁺ attacks the ambidentate bridging ligand at the end remote from the cobalt centre. Differences in activation enthalpy for the reduction of the complexes are attributed to differences in (a) the enthalpy of formation of the precursor complex and (b) the ease of stretching the Co–S or Co–N bond in the precursor complex.