Robin S. Murray

Substitution reactions of di-µ-cyano-bis[tetracyanoferrate(II)] in aqueous solution

Data obtained in a kinetic investigation of the substitution reactions of the binuclear complex ion [FeII2(CN)10]6–by X [X = HCN, pyridine (py), and 4-methylpyridine (Mepy)] support the stepwise mechanism in equations (i)–(v), [(NC)4FeII(CN)(NC)FeII(CN)4]6–+ H2O [graphic omitted] [(NC)5FeII(NC)FeII(CN)4(OH2)]6–(i),[FeII2(CN)10(OH2)]6–+ H2O [graphic omitted] 2[FeII(CN)5(OH2)]3–(ii), [FeII2(CN)10(OH2)]6–+ X [graphic omitted] [FeII2(CN)10X]+ H2O (iii), [FeII2(CN)10X]+ H2O [graphic omitted] [FeII(CN)5X]+[FeII(CN)5(OH2)]3–(iv),[FeII(CN)5(OH2)]3–+ X [graphic omitted][FeII(CN)5X](v), At 25 °C, k3=(1.25 ± 0.5 )× 10–3 s–1, k–3= 0.165 ± 0.035 s–1, k2HCN=(1.81 ± 0.07 )× 10–3 s–1, k7HCN= 34 ± 8 dm3 mol–1 s–1, k7py= 10 ± 3 dm3 mol–1 s–1, and k7MepY= 9 ± 2 dm3 mol–1 s–1; ΔH‡, ΔH2HCN‡,ΔS3‡, and ΔS2HCN‡ are 99 ± 4 kJ mol–1, 92 ± 4 kJ mol–1, 52 ± 16 J K–1 mol–1, and 31 + 12 J K–1 mol–1.

Iron(II) catalysis in substitution reactions of amminepentacyano- and aquopentacyano-ferrate(III) ions

Substitution reactions of the ions [FeIII(CN)5X]2–(X = NH3 and H2O) by Y {Y = N3–, SCN–, OH–, and [Co(CN)6]3–} are catalysed by [FeII(CN)5X]3–. At iron(II) concentrations greater than ca. 1% of the total iron species present, the rate-determining step of these reactions is substitution of the ions [FeII(CN)5X]3– by Y.

Reinvestigation of the reaction between hexacyanoferrate(III) and sulphite ions

A reinvestigation of the reduction of [Fe(CN)6]3– by SO32– has shown that the previously reported intermediate species is formed by reaction between [Fe(CN)6]3– and SO3–˙ radical ions.

Kinetics of formation and bridge cleavage of binuclear complexes derived from the pentacyanoferrate(II and III) ions in aqueous solution

Kinetic data for the reaction of [FeII(CN)5(OH2)]3–with [FeII(CN)6]4– and [CoIII(CN)6]3– to give [FeII2(CN)11]7– and [FeIICoIII(CN)11]6– respectively, and the bridge-cleavage reactions of the binuclear complexes, show that these reactions are predominantly dissociative. The bridge-cleavage reaction of [FeIIICoIII(CN)11]5– is much slower than the corresponding reaction of [FeIICoIII(CN)11]6– and the rate of the former complex is accelerated by the addition of [FeII(CN)5(OH2)]3–.

An intermediate in the reaction between trans-aquabis(ethylenediamine) sulphitocobalt(III) and sulphite ion in aqueous solution

At 25 °C. pH 7.1, and I= 1.00 mol dm–3, the substitution reaction of trans-[Co(en)2(OH2)(SO3)]+ by [SO3]2– proceeds via the O-bonded intermediate trans-[Co(en)2(OSO2)(SO3)]– as follows: [Co(en)2(OH2)(SO3)]++[SO3]2– [graphic omitted] {[Co(en)2(OH2)(SO3)][SO3]}–, {[Co(en)2(OH2(SO3)][SO3]– [graphic omitted] [Co(en)2(OSO2)(SO3)]–+ H2O, [Co(en)2(OSO2)(SO3)]– [graphic omitted] [Co(en2(SO3)2]–, The values of KIP, k1′(lim.), and k2 are 30 ±1 dm3 mol–1, 20 ± s–1, and 59 ± 7 s–1.

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

The rate of reaction (i) has been studied by stopped-flow spectrophotometry. The rate law (ii) is obeyed at [FeII(CN)5(NO)]2–+[CoIII(en)2(SO3)(OH2)]+ [graphic omitted] [(ON)(NC)4FeIICNCoIII(en)2(SO3)]–(i)-d[Co]/dt =kt[Co][Fe](ii) pH 0–6 and 25·0 °C : kt= 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 kt= 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)4FeIICNCoIII(en)2(SO3)]–+[FeIII(CN)6]3– [graphic omitted] [(NC)5FeIIICNCoIII(en)2(SO3)]2–+[FeII(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, kd= 0·113 ± 0·006 s–1[I= 1·00M(LiClO4)]. The dissociation rate constant for the complex [(NC)5CoIIICNCoIII(en)2(SO3)]2– has been obtained similarly: Kd=(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 [FeII(CN)6]4–+[CoIII(en)2(SO3)(OH)]⇌[(NC)5FeIICNCoIII(en)2(SO3)]3–+ OH–(iv) enabled kd for the dissociation of [(NC)5FeIICNCoIII(en)2(SO3)]3– to be obtained: kd=(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.

Redox decomposition of trans-tetra-ammineaquosulphitocobalt(III) in aqueous solution

The complex trans-[Co(NH3)4(H2O)(SO3)]+ decomposes to Co2+ in aqueous acid solution. Kinetic plots exhibit an initial period of curvature followed by a first-order process, kobs=(1·14 ± 0·03)× 10–4 s–1 at 25·0 °C, ΔH‡= 26·5 ± 0·1 kcal mol–1, and ΔS‡= 12·5 ± 0·4 cal K–1 mol–1[I= 1·00M(LiClO4)]. The rate of reaction is independent of [H+] in the range 0·01–1·00M. A mechanism is proposed relating kobs to a rate-determining isomerisation to the cis-complex, followed by rapid conversion to the complex [Co(NH3)3(H2O)2(SO3)]+ which probably undergoes an internal redox reaction. Somewhat higher rates have been observed in the presence of molecular oxygen and evidence is presented for formation of the free radical HSO3.

Some reactions of pentacyanosulphitoferrate(II) with oxidising agents

Oxidation of the species [FeII(CN)5(SO3)]5– by one equivalent of the oxidising agents [IrIVCl6]2– and [FeIII(CN)6]3– leads rapidly to the formation of the complex [FeIII(CN)5(SO3)]4– which reacts with an excess of [IrIVCl6]2– to form [FeIII(CN)5(OH2)]2– and [SO4]2–. Further oxidation of [FeIII(CN)5(SO3)]4– by Br2 gives a red complex (λmax. 490 nm). The observed first-order rate constants for the reactions of [FeIII(CN)5(SO3)]4– with the different oxidants are the same, and are independent of the concentration of the oxidant.

Kinetic and equilibrium studies on iron(II) and iron(III) pentacyanoferrates

The results of a kinetic investigation of the reaction of [FeII(CN)5(py)]3–(py = pyridine) with CN– and HCN are reported, together with a linear free-energy treatment of data obtained for aquation reactions of species of the type [FeII(CN)5X](X = an uncharged or univalent anion). Equilibrium studies of [FeII(CN)5OH2]3– and [FeIII(CN)5OH2]2– with a number of substituents have confirmed the class (b) or ‘soft’ character of the iron centre in these complexes.