Electron transfer at tetrahedral cobalt(II), part IV: kinetics of silver(I) catalysed chlorate reduction

Summary The very slow reduction of ClO _inf3 ^su– by [CoW₁₂O₄₀]^6- is markedly catalysed by Ag¹ in aqueous HClO₄ solution at I = 1.0 M (NaClO₄). The reaction obeys the rate law: -d[reductant]/dt where a = (1.28 ± 0.10) × 10^-4M^-1s^-1 and b = (2.22 ± 0.20) × 10^-4M^-3s^-1, at T = 60.1 ± 0.1°C and [H⁺] = 0.10–1.50 M. Activation parameters have been determined for each pathway and the reaction is discussed in terms of the outer-sphere mechanism.Part III: ref. 1.     

Stoichiometry and kinetics of the oxidation of hydroxylammonium ion by 12-tungstocobaltate(III) anion in aqueous solution

Summary The stoichiometry and kinetics of the oxidation of hydroxylammonium ion by the 12-tungstocobaltate(III) anion has been studied in hydrochloric acid medium. The ratio of mols of oxidant consumed per mol of hydroxylammonium ion is 11 and the evolution of nitrogen is confirmed. In the 0.1–1.0 mol dm^–3 [H⁺] region, the oxidation is acid-independent and obeys the empirical rate law: –d[oxidant]/dt=k[oxidant] [reductant] where k=(3.51±0.18)×10^–4 mol^–1dm³s^–1 at 22.4±0.1C and I=2.0 mol dm^–3 (NaCl). Possible reaction steps and mechanism are suggested.     

Kinetics and mechanism of ligand substitution of mono(polyamine)-nickel(II) complexes with 4-(2-pyridylazo)resorcinol

Summary The kinetics and mechanism of ligand substitution reactions of tetraethylenepentamine nickel(II), Ni (Teren), and triethylenetetraamine nickel(II), Ni(Trien), with 4-(2-pyridylazo)resorcinol (parH₂) have been studied spectrophotometrically at I=0.1 M (NaClO₄) at 25°C. In both systems two distinct reaction steps are observed. The rapid first step follows the rate law d[Ni(Polyamine)(ParH₂)]/dt=k₁ [Ni(Polyamine)] [ParH₂]. The formation of ternary complexes of Ni (Polyamine) with ParH₂ has been investigated under second order equal concentration conditions. The values of second order rate constants for the Trien and Teren reactions are (2.1±0.2)×10⁴ M^–1s^–1 and (7.8±0.6)×10³ M^–1s^–1 respectively at pH=9.0, I=0.1 M and 25°C.The rate law for the second step may be written as d[Ni(Par)₂]/dt=k₂[Ni(Polyamine)(ParH₂)]. Values of k₂ for the Trien and Teren systems are (2.5±0.1)×10^–4 s^–1 and (4.76±0.3)×10^–5 s^–1 respectively.     

Oxidation of glutathione by diaquatetrakis(2,2′-bipyridine)-μ-oxo diruthenium(III) ion in aqueous acidic solutions

Summary The kinetics of the oxidation of glutathione by diaquatetrakis(2,2-bipyridine)--oxo diruthenium(III) ion in aqueous HClO₄ have been investigated. The reaction obeys the empirical rate law:-2d[oxidant]/dt = k[oxidant][reductant]/[H⁺] where k = 7.42 ± 0.40 × 10^-3 s^-1 at 25.5 °C, [H⁺] = 0.005–0.05 M and I = 1.0 M (LiClO₄). Free radicals are important in the reaction and a mechanism consistent with the experimental results has been postulated.     

Kinetics and mechanism of the acid-catalyzed hydrolysis of [carbonatoethylenediamine(L)2cobalt(III)]+ ions

The kinetics of acid-catalyzed hydrolysis of the [Co(en)(L)₂(O₂CO)]⁺ ion (L = imidazole, 1-methylimidazole, 2-methylimidazole) follows the rate law –d[complex]/dt = {k ₁ K[H⁺]/(1 + K[H⁺])}[complex] (15–30 or 25–40 °C, [H⁺] = 0.1–1.0 M and I = 1.0 M (NaClO₄)). The reaction course consists of a rapid pre-equilibrium protonation, followed by a rate determining chelate ring opening process and subsequent fast release of the one-end bound carbonato ligand. Kinetic parameters, k ₁ and K, at 25 °C are 5.5 × 10^–2 s^–1, 0.44 M^–1 (ImH), 5.1 × 10^–2 s^–1, 0.54 M^–1 (1-Meim) and 3.8 × 10^–3 s^–1, 0.74 M^–1 (2-MeimH) respectively, and activation parameters for k ₁ are H₁ = 43.7 ± 8.9 kJ mol^–1, S₁ = –123 ± 30 J mol^–1 deg^–1 (ImH), H₁ = 43.1 ± 0.3 kJ mol^–1, S₁ = –125 ± 1 J mol^–1 deg^–1 (1-Meim) and H₁ = 64.2 ± 4.3 kJ mol^–1, S₁ = –77 ± 14 J mol^–1 deg^–1 (2-MeimH). The results are compared with those for similar cobalt(III) complexes.     

A kinetic study of the oxidation of 3- and 4-pyridinemethanol,and 3- and 4-pyridinecarboxaldehyde by chromium(VI) in acidic aqueous media

Oxidation of 3-pyridinemethanol (3-pyol), 4-pyridinemethanol (4-pyol), 3-pyridinecarboxaldehyde (3-pyal) and 4-pyridinecarboxaldehyde (4-pyal) by Cr^VI was studied under pseudo-first-order conditions in the presence of a large excess of reductant and at various H_aq ⁺ concentrations; [Cr^VI] = 8 × 10^–4 M, [reductant] = 0.025–0.20 M, [HClO₄] = 1.0 and 2.0 M (I = 1.2 and 2.1 M) or 0.5–2.0 (I = 2.1 M). A linear dependence of the pseudo-first-order rate constant (k _obs) on [reductant] and a parabolic function of k _obs versus [H⁺] lead to the rate law: –d[Cr^VI]/dt = (a + b[H⁺]²)[reductant][Cr^VI], where a and b describe the reaction paths via HCrO₄ ^– and H₃CrO₄ ⁺ species respectively, and are composite values including rate constants and equilibrium constants. The apparent activation parameters were determined from second-order rate constants at 1.0 and 2.0 M HClO₄, at three temperatures within the 293–323 K range. The presence of chromium species with intermediate oxidation states – Cr^V, Cr^IV and Cr^II, was deduced based on e.s.r. measurements and the kinetic effects of Mn^II or O₂ (Ar), respectively. The alcohols were oxidized to the aldehydes, and carboxylic acids and the aldehydes to the carboxylic acids. Chromium(III) was in the form of the [Cr(H₂O)₆]³⁺ complex.     

Kinetics of oxidation of pyruvic acid by [ethylenebis(biguanide)]silver(III) in aqueous acidic media

The oxidation of pyruvic acid by the title silver(III) complex in aqueous acidic (pH, 1.1–4.5) media is described. The reaction products are MeCO₂H and CO₂, together with a colourless solution of the Ag⁺ ion. The free ligand, ethylenebis(biguanide) is released in near-quantitative yield upon completion of the reduction. The parent complex, [Ag(H₂L)]³⁺ and one of its conjugate bases, [Ag(HL)]²⁺, participate in the reaction with both pyruvic acid (HPy) and the pyruvate anion (Py^–) as the reactive reducing species. Ag⁺ was found to be catalytically inactive. At 25.0°C, I=1.0moldm^–3, rate constants for the reactions [Ag(H₂L)]³⁺+HPy (k ₁), [Ag(H₂L)]³⁺+Py^– (k ₂), [Ag(HL)]²⁺+HPy (k ₃) and [Ag(HL)]²⁺+Py^– (k ₄) arek ₁=(94±6)×10^–5dm³mol^–1s^–1, (k ₂ K _a+k ₃ K _a1)= (1.3±0.1)×10^–5s^–1 and k ₄=(58±4)×10^–5dm³mol^–1s^–1, respectively, where K _a1is the first acid dissociation constant of the [Ag(H₂L)]³⁺ and K _a is for pyruvic acid. A comparison between the k ₁ and k ₄ values is indicative of the judgement that k ₂k ₃. A one-electron inner-sphere redox mechanism seems more justified than an outer-sphere electron-transfer between the redox partners.     

Kinetic investigation of sulfur(IV) oxidation by peroxo compounds R-OOH in aqueous solution

Summary Normal and rapid-scan stopped-flow spectrophotometry in the range of 260–300 nm was used to study the kinetics of sulfur(IV) oxidation by peroxo compounds R-OOH (such as hydrogen peroxide, R=H; peroxonitrous acid, R=NO; peroxoacetic acid, R=Ac; peroxomonosulfuric acid, R=SO ₃ ^– ) in the pH range 2–6 in buffered aqueous solution at an ionic strength of 0.5 M (NaClO₄) or 1.0 M (R=NO; Na₂SO₄). The kinetics follow a three-term rate law, rate=(k_H[H]+k_HX[HX]+k_p)[HSO ₃ ^– ][ROOH] ([H] = proton activity; HX = buffer acid = chloroacetic acid, formic acid, acetic acid, H₂PO ₄ ^– ). Ionic strength effects (I=0.05–0.5 M) and anion effects (Cl^–, ClO ₄ ^– , SO ₄ ^2– ) were not observed. In addition to proton-catalysis (k_H[H]) and general acid catalysis (k_HX[HX]), the rate constant k_p characterizes, most probably, a water induced reaction channel with k_p=k_HOH[H₂O]. It is found that k_Hf(R) with k_H(mean)=2.1·10⁷ M^–2 s^–1 at 298 K. The rate constant k_HX ranges from 0.85·10⁶ M^–2 s^–1 (HX=ClCH₂–COOH; R=NO; 293 K) to 0.47·10⁴ M^–2 s^–1 (HX=H₂PO ₄ ^– ; R=H; 298 K) and the rate constant k_p covers the range 0.2·M^–1 s^–1 (R=H) to 4.0·10⁴ M^–1 s^–1 (R=NO). LFE relationships can be established for both k_HX, correlating with the pK_a of HX, and k_p, correlating with the pK_a of the peroxo compounds R-OOH. These relationships imply interesting aspects concerning the mechanism of sulfur(IV) oxidation and the possible role of peroxonitrous acid in atmospheric chemistry. A UV-spectrum of the unstable peroxo acid ON-OOH is presented.     

The kinetics of the reaction of iodine with hydrazine as studied by pulse radiolysis

Pulse radiolysis was utilized to study the iodine — hydrazine reaction in aqueous solutions of pH3 to 7, at I^– concentrations of 0.02 to 0.34M, and a constant ionic strength of 0.35M. The reaction rate was found to be proportional to [H⁺]^–1 and [I^–]^–1. Experimental results support the assumption that the rate-determining step is the reaction of I₂ with N₂H₄ with a rate constant K1.2×10⁷ M^–1s^–1.     

Kinetics and mechanism of formation and dissociation of copper(II) and nickel(II) complexes of the Schiff base bis(acetylacetone)ethylenediimine in aqueous-organic solvent media

Summary In NH₄NO₃+NH₄OH buffered 10% (v/v) dioxan-water media (pH 7.0–8.5), thePseudo-first-order rate constant for the formation of the title complexes M(baen),i.e. ML, conforms to the equation 1/k_obs=1/k+1/(kK_o.s · T_L), where T_L stands for the total ligand concentration in the solution, K_o.s is the equilibrium constant for the formation of an intermediate outer sphere complex and k is the rate constant for the formation of the complex ML from the intermediate. Under the experimental conditions the free ligand (pK_a>14) exists virtually exclusively in the undissociated form (baenH₂ or LH₂) which is present mostly as a keto-amine in the internally hydrogen-bonded state. Although the observed formation-rate ratio k^Cu/k^Ni is of the order of 10⁵, as expected for systems having normal behaviour, the individual rate constants are very low (at 25°C, k^Cu=50 s^–1 and k^Ni=4.7×10^–4s^–1) due to the highly negative S values (–84.2±3.3 JK^–1M^–1 for CuL and –105.8±4.1 JK^–1M^–1 for NiL); the much slower rate of formation of the nickel(II) complex is due to higher H value (41.2±1.0 kJM^–1 for CuL and 78.2±1.2 kJM^–1 for NiL) and more negative S value compared to that of CuL. The K_o.s values are much higher than expected for simple outer-sphere association between [M(H₂O)₆] and LH₂ and may be due to hydrogen bonding interaction.In acid media ([H⁺], 0.01–0.04 M) these complexes M(baen) dissociate very rapidly into the [M(H₂O)₆]²⁺ species and baenH₂, followed by a much slower hydrolytic cleavage of the ligand into its components,viz. acetylacetone and ethylenediamine (protonated). For the dissociation of the complexes k_obs=k₁[H⁺]+k₂[H⁺]². The reactions have been studied in 10% (v/v) dioxan-water media and also ethanolwater media of varying ethanol content (10–25% v/v) and the results are in conformity with a solvent-assisted dissociativeinterchange mechanism involving the protonated complexes.     

Oxidations of hypophosphorus and arsenious acids by 12-tungstocobaltate(III) anion in aqueous solution

Summary The kinetics of oxidation of hypophosphorus and arsenious acids by 12-tungstocobaltate(III) anion have been studied in aqueous hydrochloric acid at constant ionic strength (I=2.0 M NaCl). The reactions obey the second-order rate law d[oxidant]/dt=2k [oxidant] [reductant]. Variation of [H⁺] in the range 0.10–1.50 M has no effect on the rates. Possible mechanistic interpretations of these observations are suggested.     

Metal ion catalysed aquation of carboxylatoamine cobalt(III) complexes. Kinetics of copper(II) catalysed aquation ofcis(diformato)bis(ethylenediamine)cobalt(III) ion

Summary The aquation ofcis-[(en)₂Co(CO₂H)₂]⁺ tocis-[(en)₂Co(OH₂)(CO₂H)]²⁺ is catalysed by Cu²⁺ and the rate equation, –d[complex]_t/dt=(k_Cu[Cu²⁺]+k_H [H⁺]) [complex)_T is valid at [Cu²⁺]_T=0.01–0.1, I=0.5 and [HClO₄]=0.005 mol dm^–3. The rate measurements are reported at 30, 35, 40 and 45°C and the rate and activation parameters for the Cu²⁺ and H⁺-catalysed paths are: k_H(35°C)=(2.44±0.09)×10^–2 dm³ mol^–1 s^–1, H=83±13 kJ mol^–1, S=–8±42 JK^–1 mol^–1, k _Cu (35°C)=(3.30±0.09)×10^–3 dm³ mol^–1 s^–1, H=73.2±6.1 kJ mol^–1, S=–55±20 JK^–1 mol^–1. The formate-bridged innersphere binuclear complex,cis-[(en)₂Co{(O₂CH)₂Cu}]³⁺ may be involved as the catalytically active intermediate in the copper(II)-catalysed path, just as the corresponding H⁺-bridged species presumed to be present in the acidcatalysed path.     

Electron-transfer kinetics and mechanism of the reduction of octacyanometallates(IV) (M = Mo,W) by hydroxide ion in aqueous solution

Summary The kinetics of the reduction of octacyanometallates(IV) in alkaline aqueous medium have been studied spectrophotometrically. The experimental results are in agreement with following rate law:-d[M(CN) _inf8 ^sup3– ]/dt = k _obs[M(CN) _inf8 ^sup3– ]²[OH^–][Na⁺] where k _obs = 4.1 × 10^–2M^–3s^–1 (Mo) and 4.0 × 10^–4 M^–3 s^–1 (W). The rate data were used to calculate the thermodynamic activation parameters H and S . A mechanism of the reaction is discussed.On leave from Faculty of Chemistry, Forest Engineering Institute, Archangelsk, Russia.     

Rates of reactions of some aqueous free radicals with platinum-ammine complexes using pulse radiolysis

The fast reaction technique of pulse radiolysis in conjunction with UV- visible absorption detection was used to determine the rate of reactions of hydrated electron, hydrogen atom, hydroxyl radical and dichloride anion radical with tetraammineplatinum(II) perchlorate and with trans- dihydroxotetraammineplatinum(IV) perchlorate complexes. Generally these reactions proceed at near diffusion-controlled rates. The second-order rate constant for the reaction of e _aq ^– , H, OH and Cl ₂ ^– radical with the Pt(II) complex are (1.9±0.1)·10¹⁰ M^–1·s^–1, (2.8±0.3)·10¹⁰ M^–1·s^–1, (6.6±0.4)·10⁹ M^–1·s^–1 and (9±1)·10⁹ M^–1·s^–1, respectively. The rate constant for the reaction of e _aq ^– with the Pt(IV) complex is (4.9±0.3)·10¹⁰ M^–1·s^–1, however, H atom and OH radical reactions proceed at relatively slower rates.     

Preparation,characterization and a kinetic study oftrans-[Ru(NH3)4L(H2O)](PF6)2 [L=PPh3, AsPh3, or SbPh3]

Summary The complexestrans-[Ru(NH₃)₄(H₂O)PPh₃](PF₆)₂ and [Ru(NH₃)₅L](PF₆)₂, (L=AsPh₃ or SbPh₃) have been isolated and characterized by microanalysis, cyclic voltammetry and ultraviolet-visible spectroscopy. The specific rate constants for the aquation of [Ru(NH₃)₅L]²⁺ totrans-[Ru(NH₃)₄L(H₂O)]²⁺ are (2.5±0.1)×10^–5s^–1 and (1.8±0.1)×10^–5s^–1 for L=AsPh₃ and SbPh₃, respectively, at 25.0±0.1°C; =0.10 mol dm^–3, NaO₂CCF₃. Under the same conditions, the second-order rate constants for the substitution of water intrans-[Ru(NH₃)₄(H₂O)L]²⁺ by isonicotinamide (isn) are 1.2±0.1, (6.3±0.3)×10^–2 and (3.8±0.2)×10^–2 m ^–1s^–1 for L=PPh₃, AsPh₃, and SbPh₃, respectively, suggesting that the order of decreasingtrans-effect is: PPh₃AsPh₃>SbPh₃. The formation constants for thetrans-[Ru(NH₃)₄L(isn)]²⁺ complexes are 75±3, (1.40±0.01)×10³ and (1.80±0.02)×10³M^–1 for L=PPh₃, AsPh₃, and SbPh₃, respectively, suggesting that the order of increasingtrans-influence is: SbPh₃3PPh₃.     

Kinetics and mechanism of the oxidation of a ternary complex involving nitrilotriacetatocobaltate(II) and malonic acid by N-bromosuccinimide

The kinetics of oxidation of [Co^IINM(H₂O)]^3– (N = nitrilotriacetate, M = malonate) by N-bromosuccinimide (NBS) in aqueous solution have been found to obey the equation: d[Co^III]/dt = k ₁ K ₂[NBS][Co^II]_T/{1 + K₂[NBS] + (H⁺/K₁)} where k ₁ is the rate constant for the electron transfer process, K ₁ the equilibrium constant for dissociation of [Co^IINM(H₂O)]^3– to [Co^IINM(OH)]^4– + H⁺, and K ₂ the pre-equilibrium formation constant. Values of k ₁ = 1.07 × 10^–3 s^–1, K ₁ = 4.74 × 10^–8 mol dm^–3 and K ₂ = 472 dm³ mol^–1 have been obtained at 30 °C and I = 0.2 mol dm^–3. The thermodynamic activation parameters have been calculated. The experimental rate law is consistent with a mechanism in which the deprotonated [Co^IINM(OH)]^4– is considered to be the most reactive species compared to its conjugate acid. It is assumed that electron transfer takes place via an inner-sphere mechanism.     

A kinetic study of the oxidation of 2-pyridinemethanol, 2-pyridinecarboxaldehyde and 2,6-pyridinedimethanol by chromium(VI) in acidic aqueous media

Oxidation of 2-pyridinemethanol (2-pyol), 2,6-pyridinedimethanol (2,6-pydol) and 2-pyridinecarboxaldehyde (2-pyal) by Cr^VI was studied under pseudo-first-order conditions in the presence of a large excess of reductant and at various H⁺ _aq concentrations; [Cr^VI] = 8 × 10^–4 M, [reductant] = 0.025–0.20 M, [HClO₄] = 1.0 and 2.0 M (I = 1.2 and 2.1 M) or 0.5–2.0 (I = 2.1 M). A parabolic dependence of the pseudo-first-order rate constant (k _obs) versus [H⁺] was observed for all the reductants. A linear dependence of k _obs on [2,6-pydol] and, unusually, higher than first-order dependence on [2-pyol] and [pyal] was established. The apparent activation parameters for reactions studied at constant [H⁺] at I = 1.2 and 2.1 M were determined. The presence of chromium species at the intermediate oxidation states: Cr^V, Cr^IV and Cr^II, was deduced based on e.s.r. measurements and the kinetic effects of Mn^II or O₂ (Ar), respectively. Comparison of the available second-order rate constants for aromatic alcohols and aldehydes demonstrated that chelating abilities of the reductant facilitates the redox process, whereas the electron-withdrawing effect caused by protonating the pyridine nitrogen atom acts in the opposite direction. The unusual low reactivity of 2-pyol was ascribed to intramolecular hydrogen bond formation.     

On the mechanism of racemisation of dichlorobis(ethylenediamine)-metal chlorides [metal = cobalt(III) or chromium(III)] in the solid state

Summary In the solid state l-cis-[M(en)₂Cl₂]Cl [M=cobalt(III) or chromium(III)] undergoes thermal racemisation smoothly at 158 °C without anycis-trans interconversion. The values of k_rac, H and S are 6 × 10^–6s^–1, 218 kJM^–1 and 156.1 JK^–1M^–1 for the cobalt(III) complex and 3.5 × 10^–5s^–1, 229.7 kJM^–1 and 197.9 JK^–1M^–1 for the chromium(III) complex, respectively. The results are only in accord with a rhombic twist mechanism of the type originally proposed by Ray and Dutt for [M(AA)₃] complexes.     

A study by pulse radiolysis of the radiation-chemical transformation of aqueous solutions of hydrazine in the presence of oxygen

It is shown by pulse radiolysis that in aqueous solutions of hydrazine containing oxygen the radical N₂H₃ reduces oxygen to O₂ ^– at pH > 7 (k 3·10⁹ dm³· mole^–1·sec^–1), while this reaction does not occur for the protonated form N₂H₄ ⁺ at pH < 7 (k, 5·10⁶ dm³·mole^–1·sec^–1). The rate constants for the disappearance of O₂ ^– have been determined in the pH range from 4 to 12. Rate constants have been calculated for the reaction of O^– with N₂H₄ [k=(1.6 ±0.2)·10⁹ dm³·mole^–1·sec^–1] and of O₃with N₂H₄ [k=(1.2 ±0.2)·10⁶ dm³· mole^–1·sec^–1].Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 341–345, February, 1991.     

Ion–molecule reaction in the gas phase 3—nucleophilic substitution of 17ξ-R-5α,14β-androstane-14,17ξ-diols under [NH4]+ chemical ionization conditions1

Under Ammonia chemical Ionization conditions the source decompositions of [M + NH₄]⁺ ions formed from epimeric tertiary steroid alchols 14 OH_β, 17OH_α or 17 OH_β substituted at position 17 have been studied. They give rise to formation of [M + NH₄? H₂O]⁺ dentoed as [MH_sH]⁺, [M_sH? H₂O]⁺, [M_sH? NH₃]⁺ and [M_sH? NH₃? H₂O]⁺ ions. Stereochemical effects are observed in the ratios [M_sH? H₂O]⁺/[M_sH? NH₃]⁺. These effects are significant among metastable ions. In particular, only the [M_sH]⁺ ions produced from trans-diol isomers lose a water molecule. The favoured loss of water can be accounted for by an S_N2 mechanism in which the insertion of NH₃ gives [M_sH]⁺ with Walden inversion occurring during the ion-molecule reaction between [M + NH₄]⁺ + NH₃. The S_N1 and S_Ni pathways have been rejected.