Roger S. Taylor

Kinetics of the rapid monomer–dimer equilibration of molybdenum(VI) in aqueous perchloric acid solutions

The monomer–dimer equilibration of molybdenum(VI) in 0·2–3·0M perchloric acid solutions, I= 3·0M(LiClO4), has been studied at 25 °C. Spectrophotometric measurements are consistent with the presence of a monomeric species, [HMoO3]+, and dimeric species, [H2Mo2O6]2+ and [H3Mo2O6]3+, in agreement with previous work by Krumenacker. Evidence for the further monomeric species [H2MoO3]2+ has been obtained. Kinetic studies using the temperature-jump technique indicate a major pathway (i) for equilibration, with rate constant K1=[HMoO3]++[HMoO3]+ [graphic omitted] [H2Mo2O6]2+(i)(1·71 ± 0·10)× 105 l mol–1s–1 and k–1=(3·20 ± 0·20)× 103s–1. The equilibration(ii), appears to contribute [H2MoO3]2++[HMoO3]+ [graphic omitted] [H3Mo2O6]3+(ii) to a small extent with k2=(0·3 ± 0·3)× 105 l mol–1s–1 and k–2=(30 ± 20) s–1.

Synthesis and characterization of the µ-disulphido complexes CrS2Cr4+ and CrS2HFe4+

Novel chromium(III) complexes with µ-di-sulphido(S–S) bridging ligands have been prepared by I2 and FeIII oxidation of CrSH2+.

The uncatalysed and mercury(II)- and thallium(III)-catalysed elimination of chloride from the µ-amido-µ-chloro-bis[tetra-amminecobalt(III)] complex

The kinetics of the mercury(II)- and thallium(III)-catalysed elimination of chloride from the title complex (I), equation (i), have been studied in aqueous HClO4 solution, I= 2·0M(NaClO4). Both reactions are independent [graphic omitted] of [H+] in the range 0·5–2·0M. With mercury(II) there is a less than first-order dependence on [Hg2+] which cannot be satisfactorily accounted for in terms of a 1 :1 adduct. Instead the dependence can be explained by considering Hg2+-catalysed conversion of (II) into (III) and applying the steady-state approximation for (II). From the treatment (at 25 °C), k1= 3·02 × 10–3 s–1, ΔH‡1= 19·2 ± 0·5 kcal mol–1, ΔS‡1=–5·7 ±1·8 cal K–1 mol–1, and k2/k–1= 0·028. At 25 °C ratios k3/k2= 1·57 × 103 I mol–1 and k4/k2= 1·3 × 103 I mol–1 are measures of the effectiveness of Hg2+ and HgCl+ as catalysts for (II)→(III). Thallium(III) produces only a mild catalytic effect, a first-order dependence on [Tl3+] is observed, and k5/k2= 15 I mol–1 is a measure of the effectiveness of Tl3+ as a catalyst. It is concluded that Tl3+ is a relatively weaker catalyst compared to Hg2+ for (II)→(III) than for the catalysed aquation of mononuclear chloro-complexes.

Reactions of µ-hydroxo-dicobalt(III) complexes. Part XIII. Further kinetic studies on the reaction of thiocyanate with the µ-amido-µ-hydroxo-bis[tetra-amminecobalt(III)] complex

Further studies have been made on the reaction of the µ-amido-µ-hydroxo-bis[tetra-amminecobalt(III)] complex with a large excess of thiocyanate, (i), in aqueous perchloric acid solutions I= 2.0 M(NaClO4). [graphic omitted] + H++ 2NCS–→ [graphic omitted] + H2O (i) Hydroxo-bridge cleavage is observed, where the concentration dependence of pseudo-first-order rate constants, kobs, at low [H+] and [NCS–] is of the form (ii), with k2 small and negligible at high [H+] and [NCS–]. kobs=k1[H+][NCS–]+k2[NCS–](ii) A non-linear dependence on hydrogen-ion concentration has to be taken into account with [H+] 0.5M. At 25 °C, k1=(4.7 ± 0.1)× 10–4 I2 mol–2 s–1, and activation parameters are ΔH‡1= 17.5 ± 0.4 kcal mol–1 and ΔS‡1=–14.7 ± 1.4 cal K–1 mol–1.

Kinetics of the 1 : 1 equilibration of thiocyanate with the molybdenum(V) aquo-dimer

Equilibration kinetic studies on the complexing of thiocyanate with the molybdenum(V) aquo-dimer [Mo2O4]2+, (I), have been carried out using the temperature-jump technique. With excess thiocyanate the reaction exhibits more than one relaxation effect, and the mechanism cannot be expressed in terms of a single process. With [Mo2O4]2+ in excess (ca. 5 : 1) only one relaxation time corresponding to the formation of the 1 : 1 complex [Mo2O4(NCS)]+ is observed, and this is independent of hydrogen-ion concentration in the range 0·5–2·0M, I= 2·0M, (LiClO4). Relaxation times, τ, can be expressed in terms of equilibrium concentrations of reactant as in (i), where rate constants K1 and k–1 for the formation and aquation of the 1 : 1 complex are, at 25 °C, (2·9 τ–1=K1([Mo2O42+]e+[NCS–]e)+K–1(i)± 0·1)× 104 l mol–1 s–1 and 120 ± 10 s–1 respectively. Activation parameters from data at 15–35 °C are ΔH1‡= 11·3 ± 0·9 kcal mol–1, ΔS1‡=–0·3 ± 3·1 cal K–1 mol–1, ΔH–1‡= 13·7 ± 2·5 kcal mol–1, and ΔS–1‡=–3·0 ± 8·0 cal K–1 mol–1. The kinetic value of the equilibrium constant (k1/k–1) at 25 °C, I= 2·0M(LiClO4), is 240 l mol–1, which is in good agreement with that obtained spectrophotometrically.