Kinetics and mechanism of pentacyanohydroxoferrate(III) formation from mono(aminocarboxylato)iron(III) complexes

Summary The kinetics and mechanism of the system: [FeL(OH)]^2–n + 5 CN^– [Fe(CN)₅(OH)]^3– + L^n–, where L=DTPA or HEDTA, have been investigated at pH= 10.5±0.2, I=0.25 M and t=25±0.1 C.As in the reaction of [FeEDTA(OH)]^2–, the formation of [Fe(CN)₅(OH)]^3– through the formation of mixed ligand complex intermediates of the type [FeL(OH)(CN)_x]^2–n–x, is proposed. The reactions were found to consist of three observable stages. The first involves the formation of [Fe(CN)₅(OH)]^3–, the second is the conversion of [Fe(CN)₅(OH)]^3– into [Fe(CN)₆]^3– and the third is the reduction of [Fe(CN)₆]^3– to [Fe(CN)₆]^4– by oxidation of L^n– The first reaction exhibits a variable order dependence on the concentration of cyanide, ranging from one at high cyanide concentration to three at low concentration. The transition between [FeL(OH)]^2–n and [Fe(CN)₅(OH)]^3– is kinetically controlled by the presence of four cyanide ions around the central iron atom in the rate determining step. The second reaction shows first order dependence on the concentration of [Fe(CN)₅(OH)]^3– as well as on cyanide, while the third reaction follows overall second order kinetics; first order each in [Fe(CN)₆]^3– and L^n–, released in the reaction. The reaction rate is highly dependent on hydroxide ion concentration.The reverse reaction between [Fe(CN)₅(OH)]^3– and L^n– showed an inverse first order dependence on cyanide concentration along with first order dependence each on [Fe(CN)_5– (OH)]^3– and L^n–. A five step mechanism is proposed for the first stage of the above two systems.     

Chemical control of competing electron transfer pathways in iron tetracyano-polypyridyl photosensitizers

Photoinduced intramolecular electron transfer dynamics following metal-to-ligand charge-transfer (MLCT) excitation of [Fe(CN)₄(2,2′-bipyridine)]²⁻ (1), [Fe(CN)₄(2,3-bis(2-pyridyl)pyrazine)]²⁻ (2) and [Fe(CN)₄(2,2′-bipyrimidine)]²⁻ (3) were investigated in various solvents with static and time-resolved UV-Visible absorption spectroscopy and Fe 2p3d resonant inelastic X-ray scattering (RIXS). This series of polypyridyl ligands, combined with the strong solvatochromism of the complexes, enables the ¹MLCT vertical energy to be varied from 1.64 eV to 2.64 eV and the ³MLCT lifetime to range from 180 fs to 67 ps. The ³MLCT lifetimes in 1 and 2 decrease exponentially as the MLCT energy increases, consistent with electron transfer to the lowest energy triplet metal-centred (³MC) excited state, as established by the Tanabe–Sugano analysis of the Fe 2p3d RIXS data. In contrast, the ³MLCT lifetime in 3 changes non-monotonically with MLCT energy, exhibiting a maximum. This qualitatively distinct behaviour results from a competing ³MLCT → ground state (GS) electron transfer pathway that exhibits energy gap law behaviour. The ³MLCT → GS pathway involves nuclear tunnelling for the high-frequency polypyridyl breathing mode (hν = 1530 cm⁻¹), which is most displaced for complex 3, making this pathway significantly more efficient. Our study demonstrates that the excited state relaxation mechanism of Fe polypyridyl photosensitizers can be readily tuned by ligand and solvent environment. Furthermore, our study reveals that extending charge transfer lifetimes requires control of the relative energies of the ³MLCT and the ³MC states and suppression of the intramolecular distortion of the acceptor ligand in the ³MLCT excited state.

Photoinduced intramolecular electron transfer in Fe tetracyano-polypyridyl complexes was investigated with static and time-resolved UV-visible absorption and resonant inelastic X-ray scattering which revealed a competition of two relaxation pathways.     

Syntheses,crystal structures and magnetic properties of two cyano-bridged two-dimensional assemblies [Fe(salpn)]2 [Fe(CN)5NO] and [Fe(salpn)]2[Ni(CN)4]

Two cyano-bridged assemblies, [Fe^III(salpn)]₂[Fe^II(CN)₅NO] (1) and [Fe^III (salpn)]₂[Ni^II(CN)₄] (2) [salpn = N, N-1,2-propylenebis(salicylideneiminato)dianion], have been prepared and structurally and magnetically characterized. In each complex, [Fe(CN)₅NO]^2– or [Ni(CN)₄]^2– coordinates with four [Fe(salpn)]⁺ cations using four co-planar CN^– ligands, whereas each [Fe(salpn)]⁺ links two [Fe(CN)₅NO]^2– or [Ni(CN)₄]^2– ions in the trans form, which results in a two-dimensional (2D) network consisting of pillow-like octanuclear [—M^II—CN—Fe^III—NC—]₄ units (M = Fe or Ni). In complex (1), the NO group of [Fe(CN)₅NO]^2– remains monodentate and the bond angle of Fe^II—N—O is 180.0°. The variable temperature magnetic susceptibilities, measured in the 5–300 K range, show weak intralayer antiferromagnetic interactions in both complexes with the intramolecular iron(III)iron(III) exchange integrals of –0.017 cm^–1 for (1) and –0.020 cm^–1 for (2), respectively.     

Kinetic and mechanism of pentacyanohydroxoferrate(III) formation from the reaction of [Fe2HPDTA(OH)2] with cyanide ions

Summary The kinetics and mechanism of exchange of HPDTA in [Fe₂HPDTA(OH)₂] with cyanide ion (HPDTA=2-hydroxytrimethylenediaminetetraacetic acid) was investigated spectrophotometrically by monitoring the peak at 395 nm ( _max of [Fe(CN)₅OH]^3– at pH=11.0±0.02,I=0.25m (NaClO₄) at ±0.1°C).Three distinct observable stages were identified; the first is the formation of [Fe(CN)₅OH]^3–, the second the formation of [Fe(CN)₆]^3– from it and the third the reduction of [Fe(CN)₆]^3– to [Fe(CN)₆]^4– by HPDTA^4– released in the first stage.The first stage follows first-order kinetics in [Fe₂HPDTA(OH)₂] and second-order in [CN^–] over a wide range of [CN^–], but becomes zero order at [CN^–]<5×10^–2 m. We suggest a cyanide-independent dissociation of [Fe₂HPDTA)(OH)₂] into [FeHPDTA(OH)] and [Fe(OH)]²⁺ at low cyanide concentrations and a cyanide-assisted rapid dissociation of [Fe₂HPDTA(OH)₂] to [FeHPDTA(OH)(CN)]^3– and [Fe(OH)]²⁺ at higher cyanide concentrations. The excess of cyanide reacts further with [FeHPDTA(OH)(CN)]^3– finally to form [Fe(CN)₅OH]^3–.The reverse reaction between [Fe(CN)₅OH]^3– and HPDTA^4– is first-order in [Fe(CN)₅OH]^3– and HPDTA^4–, and exhibits inverse first-order dependence on cyanide concentration.A six-step mechanism is proposed for the first stage of reaction, with the fifth step as rate determining.     

Kinetics and mechanism of pentacyanohydroxoferrate(III) formation from the reaction of cyanide ion with [Fe2L(OH)2]2− (L=triethylenetetraaminehexaacetate)

Summary The kinetics and mechanism of the reaction between [Fe₂L(OH)₂]^2– and cyanide ion (L = TTHA, triethylenetetraaminehexaacetate) have been studied spectrophotometrically atpH=11.0±0.1,I=0.1 M(NaClO₄) and T = 25±0.1 °C. The overall reaction consists of three distinct, observable stages. The first stage involves the dissociation of the binuclear complex into a mononuclear complex [FeL(OH)]^4– which then reacts with cyanide to form [Fe(CN)₅OH]^3–. The species [Fe(CN)₅OH]^3– reacts further with an excess of cyanide and forms [Fe(CN)₆]^3– in the second stage of reaction. The last stage involves the reduction of [Fe(CN)₆]^3– formed in the second stage by the TTHA^6– released in the first stage of reaction. The formation of [Fe(CN)₅OH]^3– in the first stage is firstorder in [Fe₂L(OH)₂]^2– and third-order in cyanide over a large range of cyanide concentrations but becomes zero-order in cyanide at [CN^–] < 4×10^–2M.These observations enable us to suggest the presence of a slow step in which [Fe₂L(OH)₂]^2– dissociates into [FeL(OH)]^4– and [FeOH]²⁺ at low cyanide concentrations and a cyanide assisted rapid dissociation of [Fe₂L(OH)₂]^2– to [FeL(OH)(CN)]^5– at higher cyanide concentrations. The species [FeL(OH)(CN)]^5– reacts further with an excess of cyanide to produce [Fe(CN)₅OH]^3– finally.The reverse reaction between [Fe(CN)₅OH]^3– and TTHA^6– follows first-order dependence in each of [Fe(CN)₅OH]^3– and TTHA^6– and inverse first-order dependence on cyanide concentration. A six-step mechanism has been proposed for the first stage of reaction in which the fifth has been identified as the rate-determining step.     

Synthesis,crystal structure and magnetic properties of the first dinuclear copper(II)–iron(II) complex [Cu(L)Fe(CN)5NO] based on nitroprusside

The nitrosyl cyanide [Cu(L)Fe(CN)₅NO] was prepared by the reaction of [Cu(L)]Cl₂ [L = 3, 10-bis(2-hydroxymethyl)-1,3,5,8,10,12-hexaazacyclotetradecane] with Na₂[Fe(CN)₅NO]·2H₂O in aqueous solution. Single-crystal analysis revealed that the title complex is the first structurally characterized dinuclear copper(II)–iron(II) complex based on the nitroprusside. Variable temperature magnetic susceptibility measurements (4.0–180.0 K) show the occurrence of very weak antiferromagnetic interactions between the copper(II) ions with zJ = –0.410 cm^–1.     

Dinuclear complexes with a μ-cyano ligand. Part XI(1). Synthesis and characterization of several derivatives ofcis-[Co(NH3)(en)2(H2O)]3+

Summary The new complex double saltscw-[Co(NH₃)(en)₂(H₂O)]₂ [M(CN)₄]₃ (en = ethylenediamine; M = Ni, Pd or Pt),cis-[Co(NH₃(en)₂(H₂O)]₂[FeNO(CN)₅]₃ andcis-[Co(NH₃)(en)₂(H₂O)][Co(CN)₆] have been synthesized and by anation in the solid state the corresponding new dinuclear complexes with a cyano bridgecis- ortrans-[(NH₃)(en)₂Co-NC-M(CN)₃]₂ [M(CN)₄] (M = Ni, Pd or Pt);cis-, trans-[(NH₃)(en)₂Co-NC-FeNO(CN)₄]₂[FeNO(CN)₅] andcis-[(NH₃)(en)₂Co-NC-Co(CN)₅ have been prepared. The complexes have been characterized by chemical analysis, t.g. measurements, and by i.r. and electronic spectroscopy. With [Ni(CN)₄][^2– and [Co(CN)in]₆ ^3– only thecis-isomer is produced; with [Pd(CN)₄]^2–, [Pt(CN)₄]^2– and [FeNO(CN)₅]^2– thetrans- isomer is the dominant species. The dinuclear complex derived from [Pt(CN)₄]^2– shows strong Pt-Pt interactions both in the solid state and in solution.     

Kinetics and mechanism of pentacyanohydroxoferrate(III) formation from [FeL(OH)2]− complex, (L=N-(2-hydroxyethyl) iminodiacetate anion

Summary The kinetics and mechanism of the system [FeHIDA-(OH)₂]^–+5CN^–[Fe(CN)₅OH^–+HIDA^2–+OH^– (HIDA=N-(2-hydroxyethyl) (iminodiacetate) at pH=9.5±0.02, I=0.1 M and at 25±0.1°C have been studied spectrophotometrically at 395 nm ( _max of [Fe(CN)₅OH]^3–]. The reaction has three distinguishable stages; the first is formation of [Fe(CN)₅OH]^3–, the second is conversion of [Fe(CN)₅OH]^3– into [Fe(CN)₆]^3–, and last is the reduction of [Fe(CN)₆]^3– to [Fe(CN)₆]^4– by the HIDA^2– released in the first stage. The first stage shows variable-order dependence on cyanide concentration, unity at high cyanide concentration and zero at low cyanide concentration. The second stage exhibits first-order dependence on the concentration of [Fe(CN)₅OH]^3– as well as on cyanide. The reverse reaction between [Fe(CN)₅OH]^3– and HIDA^2– is first-order in each of these species and inverse first-order in cyanide. On the basis of forward and reverse rate studies, a five-step mechanism has been proposed for the first stage. The first step involves a slow loss of one OH^–, by a cyanide-independent path.     

The limiting dissociative mechanism for substitution at thed 6 centres molybdenum(O), iron(II), and cobalt(III): Kinetics of substitution at the pentacyano-4-Cyanopyridineferrate(II) anion and at 4-cyanopyridinemolybdenum(I) pentacarbonyl

Summary Rate constants are reported for reaction of the 4-cyanopyridine complexes [Fe(CN)₅(4CNpy)]^3– and [Mo(CO)₅(4CNpy)] with a variety of incoming ligands, in aqueous methanol (40 vol % MeOH) and in toluene respectively, at 298.2 K (ambient pressure). The dependence of rate constants on the nature and concentration of the incoming ligand is discussed in terms of the operation of the limiting dissociative,D, mechanism for substitution; the operation of this mechanism here, and in analogous pentacyanoferrate(II), pentacarbonylmolybdenum(I), and penta- and tetra-cyanocobaltate(III) complexes is reviewed. The effect of pressure on rate constants for replacement of 4-cyanopyridine in [Mo(CO)₅(4CNpy)], in toluene solution at 298.2 K, indicates an activation volume of +3 cm³ mol^–1.     

Ligand substitution and redox properties of 4-picolylaminepentacyanoferrate(II)

Summary The stability constant for [Fe^II(CN)₅(4-picam)]^3– (4-picam=4-picolylamine), K₂, was calculated from the measured dissociation and formation constants. The reduction potential obtained by cyclic voltammetry allowed the calculation of K₃. Comparison of K₂ and K₃ for this and other substituted pentacyanoferrate(II) complexes supports the fact that bonding properties of azines coordinated to {Fe(CN)₅}^n– change with the iron oxidation state. The variation on the rate of release of 4-picam with pH and added electrolyte concentration was interpreted on the basis of solvent effects.Predoctoral fellow of CONICET, República Argentina.Member of the Carrera del Investigador Científico, CONICET, República Argentina.     

Rate constant for water loss from low-spin iron(II) in aquapentacyanoferrate(II)

Rate constants for the reaction of [Fe(CN)₅(H₂O)]^–3 with [Co(NH₃)₅(pyrazine)]³⁺ have been analysed on the basis of a pre-association equilibrium constant and a rate constant for the subsequent ligand interchange. The latter represents an unusual parameter, a rate constant for water loss from a low-spin iron(II) centre.     

Photochemical reactions of molybdenum and tungsten octacyanometallates(IV) in acidic aqueous media containing 2,2′-bipyridyl or 1,10-phenanthroline

Summary The reaction scheme of acidic photolysis of [M(CN)₈]^4– (M = Mo or W) in the presence of 2,2-bipyridyl (bipy) or 1,10-phenanthroline (phen) based on previous reports, and the present results, is given. In this scheme the formation of [M(CN)₆(N-N)]^2– (M = Mo or W), postulated in the literature to be a main product of photoexcitation of [M(CN)₈]^4– in the presence of bipy or phen, has definitively been excluded. The main cyano-polypyridyl species formed are [MO(CN)₃(N-N)]^– ions which, in acidic solution, undergo further reactions. A new product, [MoO(CN)₂(N-N)₂], resulting from thermal replacement of the cyanide ligand by polypyridyl, has been detected.Author to whom all correspondence should be directed.     

Mechanistic aspects of the reduction of ruthenium(III) catalyzed

The kinetics of Ru_III catalyzed reduction of hexacyanoferrate(III) [Fe(CN)₆]^3–, by atenolol in alkaline medium at constant ionic strength (0.80 mol dm^–3) has been studied spectrophotometrically, using a rapid kinetic accessory. The reaction between atenolol and [Fe(CN)₆]^3– in alkaline medium exhibits 1:2 stoichiometry [atenolol:Fe(CN)₆ ^3–]. The reaction showed first order kinetics in [Fe(CN)₆]^3– concentration and apparent less than unit order dependence, each in atenolol and alkali concentrations. Effect of added products, ionic strength and dielectric constant of the reaction medium have been investigated. A retarding effect was observed by one of the products i.e., hexacyanoferrate(II). The main products were identified by i.r., n.m.r., fluorimetric and mass spectral studies. A mechanism involving the formation of a complex between the atenolol and the hydroxylated species of ruthenium(III) has been proposed. The active species of oxidant and catalyst were [Fe(CN)₆]^3–and [Ru (H₂O)₅OH]²⁺, respectively. The reaction constants involved in the mechanism were evaluated. The activation parameters were computed with respect to the slow step of the mechanism, and discussed.     

Syntheses,crystal structures and magnetic properties of two honeycomb-layered bimetallic assemblies,K2[NiII(cyclam)]3[FeII(CN)6]2 · 12H2O and [NiII(cyclam)]3[FeIII(CN)6]2 · 16H2O

Two bimetallic assemblies, K₂[Ni^II(cyclam)]₃[Fe^II(CN)₆]₂ · 12H₂O (1) and [Ni^II(cyclam)]₃[Fe^III(CN)₆]₂ · 16H₂O (2) (cyclam = 1,4,8,11-tetraazacyclotetradecane), were obtained by reaction of K₄[Fe(CN)₆] and [Ni(cyclam)](ClO₄)₂ in aqueous media at different temperatures. Their crystals were structurally determined and magnetic properties were studied. Both of the compounds have honeycomb-layered structures, which are formed by Fe₆Ni₆ units linked through the cyanide bridges. Structure (1) consists of polyanions containing Ni^II–NC–Fe^II linkages and K⁺ cations, while structure (2) is a two-dimensional neutral layer containing Ni^II–NC–Fe^III linkages. The magnetic properties of (1) and (2) have been investigated in the 5–300 K range. Compound (1) exhibits a weak antiferromagnetic interaction with Weiss constant = –0.35 K; compound (2) shows ferromagnetic intralayer and antiferromagnetic interlayer interactions.     

Azido-, Rhodano-, Cyano- und Fluorokomplexe des Fe(III)-Ions in Dimethylsulfoxid

Zusammenfassung Auf Grund spektrophotometrischer und konduktometrischer Messungen wurden folgende Koordinationsformen des Eisen(III)-ions mit Azid-, Rhodanid-, Cyanid- und Fluoridionen in Dimethylsulfoxid festgestellt: [Fe(N₃)₄]^–, [Fe(SCN)₆]^3–, [Fe(CN)₂]⁺, Fe(CN)₃, [Fe(CN)₄]^–, [FeF₂]⁺, [FeF₄]^–.

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By means of spectrophotometric and conductometric measurements the following coordination forms of iron(III) with azide-, thiocyanate-, cyanide- and fluoride ions were found in dimethyl sulfoxide: [Fe(N₃)₄]^–, [Fe(SCN)₆]^3–, [Fe(CN)₂]⁺, Fe(CN)₃, [Fe(CN)₄]^–, [FeF₂]⁺, [FeF₄]^–.

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Mit 4 Abbildungen     

Synthesis,crystal structure and magnetic properties of a two-dimensional mixed-valence assembly [Fe(salen)]2[Fe(CN)5NO]

A cyanide-bridged Fe^III–Fe^II mixed-valence assembly, [Fe^III(salen)]₂[Fe^II(CN)₅NO] [salen = N,N-ethylenebis(salicylideneiminato)dianion], prepared by slow diffusion of an aqueous solution of Na₂[Fe(CN)₅NO] · 2H₂O and a MeOH solution of [Fe(salen)NO₃] in an H tube, has been characterized by X-ray structure analysis, i.r. spectra and magnetic measurements. The product assumes a two-dimensional network structure consisting of pillow-like octanuclear [—Fe^II—CN—Fe^III—NC—]₄ units with dimensions: Fe^II—C = 1.942(7) Å, C—N = 1.139(9) Å, Fe^III—N = 2.173(6) Å, Fe^II—C—N = 178.0(6)°, Fe^III—N—C = 163.4(6)°. The Fe^II—N—O bond angle is linear (180.0°). The variable temperature magnetic susceptibility, measured in the 4.8–300 K range, indicates the presence of a weak intralayer antiferromagnetic interaction and gives an Fe^III–Fe^III exchange integral of –0.033 cm^–1.     

Synthesis,protonation and substitution behaviour of K3[ReO2(CN)4]

Summary The preparation and characterization of salts of the [ReO₂(CN)₄]^3–, [ReO(OH)(CN)₄]^2–, [ReO(H₂O)(CN)₄]^–, [Re₂O₃(CN)₈]^4– and [ReO(NCS)(CN)₄]^2– species are described. The nature of the protonation reactions of [ReO₂(CN)₄]^3– was established by the successful isolation of these salts.     

Thermal and photochemical interaction of octacyanometallates(IV), [M(CN)8]4− (M = Mo or W) in the presence of pyrazine

The interactions between [M(CN)₈]^4– (M = Mo or W) and pyrazine (pz) in the solid state and in aqueous solutions have been analysed. In strongly acidic solutions {pzH⁺, [M(CN)₈]^4–} ion pair formation is observed; the pyrazinium salts (pzH)₂(H₃O)₂[Mo(CN)₈]·0.5pz·3H₂O and (pzH)₂K(H₃O)[W(CN)₈]·H₂O have been isolated. The X-ray crystal structure of the latter, and the spectroscopic properties of both, are described. The [W(CN)₈]^4– anion is approximately square antiprismatic (D_4d), with different H-bond environments around the N atoms. The ligand-field photolysis of [M(CN)₈]^4– in the presence of pyrazine in neutral and alkaline solution results in the formation of tetracyanooxometallates(IV) in equilibrium with pentacyanooxometallates(IV) through the [M(CN)₇(pz)]^3– anions as intermediates. The formation of the [M(CN)₆(pz)₂]^2– ion, postulated in the literature to be the final product of the alkaline photolysis, has definitively been excluded.     

Solvatochromism and piezochromism of metal→ligand chargetransfer bands of ternary diimine complexes of tungsten(0), molybdenum(0), and iron(II)

Summary The solvatochromic behaviour of W(CO)₄(LL), LL=a diazabutadiene or a pyridine Schiff base, and of Mo(CO)₄ (butane-2,3-dionedihydrazone) is described. Pressure effects on the charge-transfer spectra of these compounds, the [Mo(CO)₄(fz)]^2– anion {fz=ferrozine; 3-(2-pyridyl)-5, 6-bis-(4-sulphonatophenyl)-1,2,4-triazine}, and Fe(bipy)₂(CN)₂ are reported. Solvent sensitivities are discussed in terms of the nature of the ligands and of the central metal atom, and the idea of a correlation between these solvent sensitivities and piezochromic behaviour developed for these and related ternary irondiimine-cyanide species.Author to whom all correspondence should be directed.     

The formation of an antitubercular complex [Fe(CN)5(INH)]3− through mercury(II)‐catalyzed ligand substitution reaction: A kinetic and mechanistic study

The kinetics and mechanism of the formation of an antitubercular complex [Fe(CN)₅(INH)]^3? based on the substitution reaction between K₄[Fe(CN)₆] and isoniazid (INH), i.e., isonicotinohydrazide, catalyzed by Hg²⁺ in aqueous medium was studied spectrophotometrically at 435 nm (the λ_max of the golden‐yellow‐colored complex [Fe(CN)₅(INH)]^3?) as a function of pH, ionic strength, temperature, and the concentration of the reactants and the catalyst. The replacement of coordinated CN^? in [Fe(CN)₆]^4? was facilitated by incoming ligand INH under the optimized reaction conditions: pH 3.5 ± 0.02, temperature = 30.0 ± 0.1°C, and ionic strength I = 0.05 M (KNO₃). The stoichiometry of the reaction and the stability constant of the complex ([Fe(CN)₅(INH)]^3?) have been established as 1:1 and 2.10 × 10³ M, respectively. The rate of catalyzed reaction was found to be slow at low pH values, to increase with increasing pH, to attain a maximum value at 3.50 ± 0.02, and finally to decrease after pH > 3.5 due to less availability of H⁺ ions needed to regenerate the catalytic species. The initial rates were evaluated for each variation from the absorbance versus time curves. The reaction was found to be pseudo‐first order with respect to [INH] and first order with respect to [Fe(CN)₆^4?] at lower concentration, whereas it was found to be fractional order at higher [INH] and [Fe(CN)₆^4?]. The ionic strength dependence study showed a negative salt effect on the rate of the reaction. Based on experimental results, a mechanism for the studied reaction is proposed. The rate equation derived from this mechanism explains all the experimental observations. The evaluated values of activation parameters for the catalyzed reaction suggest an interchange dissociative (I_d) mechanism. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 398–406, 2012