Mixed iron–cobalt binuclear complexes. Part II. Kinetics of formation and dissociation of binuclear complexes obtained from trans-aquabis(ethylenediamine)sulphitocobalt(III) and cyano-complexes of iron

Abstract

The rate of reaction (i) has been studied by stopped-flow spectrophotometry. The rate law (ii) is obeyed at [Fe^II(CN)₅(NO)]^2–+[Co^III(en)₂(SO₃)(OH₂)]⁺ [graphic omitted] [(ON)(NC)₄Fe^IICNCo^III(en)₂(SO₃)]^–(i)-d[Co]/dt =k_t[Co][Fe](ii) pH 0–6 and 25·0 °C : k_t= 700 ± 15 I mol^–1s^–1;ΔH^‡= 59·8 ± 0·4 kJ mol^–1; and ΔS^‡= 10·5 ± 2·1 J k^–1 mol^–1[I= 1·00M(LiClO₄)]. With [Fe(CN)₆]^3– and [Fe(CN)₆]^4– as reactants, rate parameters are k_t= 1 725 ± 35 and 7 450 ± 220 I mol^–1s^–1, ΔH^‡= 60·7 ± 0·8 and 56·9 ± 1·3 kJ mol^–1, and ΔS^‡= 21·3 ± 2·5 and 19·7 ± 3·3 J K^–1 mol^–1, respectively. The rate of the reverse of reaction(i) has been measured by following the displace [(ON)(NC)₄Fe^IICNCo^III(en)₂(SO₃)]^–+[Fe^III(CN)₆]^3– [graphic omitted] [(NC)₅Fe^IIICNCo^III(en)₂(SO₃)]^2–+[Fe^II(CN)₅(NO)]^2–(iii) ment reaction (iii) which is first order in the concentration of the reacting binuclear complex and independent of the [Fe(CN)₆]^3– concentration : at 25·0 °C, k_d= 0·113 ± 0·006 s^–1[I= 1·00M(LiClO₄)]. The dissociation rate constant for the complex [(NC)₅Co^IIICNCo^III(en)₂(SO₃)]^2– has been obtained similarly: K_d=(3·41 ± 0·03)× 10^–2 s^–1 under the same conditions. A study of the rate of attainment of equilibrium (iv) in alkaline solution has [Fe^II(CN)₆]^4–+[Co^III(en)₂(SO₃)(OH)]⇌[(NC)₅Fe^IICNCo^III(en)₂(SO₃)]^3–+ OH^–(iv) enabled k_d for the dissociation of [(NC)₅Fe^IICNCo^III(en)₂(SO₃)]^3– to be obtained: k_d=(2·56 ± 0·12)× 10^–3 s^–1 at 25·0 °C [I= 1·00M(NaClO₄)]. Equilibrium constants for the formation of these binuclear complexes have been calculated from the kinetic data.