Smiljko Ašperger

Mechanism of reaction of the binuclear dimer of [Fe(CN)5(OH2)]3– with uncharged ligands in aqueous solution

The kinetics of reactions of the binuclear dimer of [Fe(CN)5(OH2)]3– with pyridine (py), nitrosobenzene (PhNO), 3CN-py, and 4CN-py, respectively, have been measured at pH 5.5–6.0. With a 20-fold, or larger, excess of the complexing ligand (no extra salt added) the average kobs. at 25 °C for all reagents used is 0.024 ± 0.001 s–1. The activation parameters for the reaction of the dimer with 3CN-py are ΔH‡= 70.3 ± 3.0 kJ mol–1 and ΔS‡= 29.7 ± 4.0 J K–1 mol–1. Ratios of rate constants (competition ratios) for the reactions of various ligand pairs (Yl and Y2) with [Fe(CN)5(OH2)]3– and its binuclear dimer have been determined. The competition ratios at 25 °C with Y1,Y2 defined as (a) py,PhNO, (b) 3CN-py,4CN-py, (c) 4CN-py,py, (d) 4CN-py,PhNO, and (e) 3CN-py,PhNO are: for the binuclear dimer, (a) 1.35, (b) 1.04, (c) 1.61, (d) 2.17, (e) 2.21; for [Fe(CN)5(OH2)]3–, (a) 1.32, (b) 1.01, (c) 1.66, (d) 2.15, (e) 2.19. Almost equal competition ratios for identical pairs Y1,Y2 for the dimer and [Fe(CN)5(OH2)]3– suggest that the aqua-complex is the only intermediate in the replacements involving the binuclear complex, and that the binuclear complex is singly bridged, having the structure [(NC)5Fe(µ-NC)Fe-(CN)4(OH2)]6–.

Interaction between hexacyanoferrate(II) ion and mercury(II) and silver(I) ions

The interaction between hexacyanoferrate(II) ion and mercury(II) or silver(I) ion was studied conductometrically in aqueous solution. The maximum decrease in conductivity occurred at a 1 : 2 mole ratio of K4[Fe(CN)6] : HgCl2 or at a ca. 1 : 4 mole ratio of K4[Fe(CN)6] : AgNO3. The decrease in conductivity is considered to be due to complex formation between hexacyanoferrate(II) ion and the metal ions. The salt effect on the HgII-catalysed dissociation of [Fe(CN)6]4– to [Fe(CN)5H2O]3– is positive at low, but negative at high, ionic strength. The mechanism of this reaction is discussed.

Reactions of cobalt(II) protoporphyrin IX dimethyl ester, [COIIP], and [CoIIIP(Cl)] in co-ordinating aliphatic alcohols

Chlorocobalt(III) protoporphyrin IX dimethyl ester, [CoIIIP(Cl)], releases its chloride immediately on dissolution in methanol, and equilibrates as shown below. Species (2) predominates if no acid [graphic omitted] or alkali is added. [CoIIP] dissolved in methanol, in the presence of air, undergoes oxidation yielding (2). The identities of the products obtained in both ways were established by comparison of absorption spectra and rates of replacements of their axial ligands. Kinetics of the reactions of (2) and various ligands L in methanol (L = pyridine, substituted pyridine, or nicotinamide) show that methanol is replaced first, and methoxide second, both in a dissociative manner. The replacements of methanol by L attain a limiting rate which does not depend on L [kobs.(25 °C)= 57.8 s–1]. The existence of a limiting rate here reveals that a D mechanism [SNl(lim.)] operates, whereas an Id mechanism is excluded. This case appears to be a unique example of a fully established D mechanism in a polar, co-ordinating solvent. Values of kobs. for the slower replacement of methoxide by L show a linear dependence on [L], with small differences in slopes for different entering ligands. This excludes a bimolecular mechanism, and suggests that the replacements of methoxide by L take place via the complex conjugate acid in a dissociative manner; the kinetic results do not allow a distinction between D and Id mechanisms.

Kinetics and mechanism of replacements in pentacyano(ligand)ferrate(II) ions

The kinetics of replacement of the ligand in the pentacyano(ligand)ferrate(II) ions have been examined for the leaving ligands NN-dimethyl(p-nitroso)aniline, nitrosobenzene, sulphite, and water, respectively, and for the entering ligands nitrosobenzene, 3-cyanopyridine. NN-dimethyl(p-nitroso)aniline, thiocyanate, nitrite, cyanide, and sulphite. Limiting reaction rates, at sufficiently large concentrations, of entering ligand have been observed with all the leaving ligands, except water, where the replacements obey the second-order rate law –d[Fe(CN)5-OH23–]/dt=kY[Fe(CN)5OH23–][Y]. When the entering ligand Y bears no electrical charge, the kY values are very similar and in the range 200–300 I mol–1 s–1 at 25 °C and 1M ionic strength. For singly negatively charged anions kY≃ 40–60, and for the doubly charged SO32– ion kY= 3·3 I mol–1 s–1. The variations in kY are interpreted as being due to variations in diffusion rates since the reactions of the intermediate [Fe(CN)5]3–with the ligands are diffusion controlled.

Acetolysis of ferrocenylmethyl benzoate. High secondary α-deuterium kinetic isotope effect for primary carbon-oxygen cleavage

Abstract Secondary α-deuterium kinetic isotope effect ( KIE ) in acetolysis of ferrocenyl-1,1-dideuteriomethyl benzoate was determined at 298 K as k H/ k D = 1.50 (22.5% per D). This appears to be one of the largest KIE observed for carbon-oxygen cleavage. The solvolysis exhibits a ‘special salt effect’, and a common ion rate depression effect, indicating the presence of solvent-separated ion pairs and the return to tight pairs. The high value of KIE in acetolysis strongly suggests that the acetolysis is a limiting dissociative process with the carbonium ion transition state, stabilized mainly by conjugation with the π-system. The entropy of activation (−30 J K −1 mol −1 ) is very similar to those values we previously determined in ethanolyses of ferrocenylmethyl acetate and benzoate (−22 and −25 J K −1 mol −1 ), where the KIE was only 11.4% per D. In the latter reactions the stabilization of the transition state probably involves some Fe-C exo bond formation. It appears that the mode of stabilization of the ferrocenylmethyl cation depends on the type of solvolysis and on the nature of the media: high KIE is observed in solvents of high ionizing power. Entropies of activation cannot help in distinguishing between the two possible modes of stabilization.

Aquation of penta-ammine(dimethyl sulphoxide)cobalt(III) perchlorate in water–non-aqueous solvent mixtures

The aquation rate constant of penta-ammine(dimethyl sulphoxide) cobalt(III) ion in various aqueous and aqueous–non-aqueous solvent mixtures has been determined. The rate constant does not correlate well with water activity, the Grunwald–Winstein solvating-power Y parameter, or the heat of mixing of dimethyl sulphoxide with the solvent components. The data support an Id mechanism more readily than a D mechanism. The activation enthalpy and entropy are 23·9 ± 0·7 kcal mol–1 and 0·36 ± 2·29 cal K–1 mol–1, respectively.

Participation of the ferrocenyl group in the solvolysis of ferrocenylmethyltrimethylammonium ion[1]

Abstract The α-deuterium kinetic isotope effect in the solvolysis of ferrocenyl-1,1-dideuteriomethyltrimethylammonium iodide in aqueous solution has been measured. The isotope effect, k H / k D , is 1.06 ± 0.04 at 80°C (about 3% per deuterium atom). This effect is only a small fraction of the limiting α-deuterium isotope effect for a nitrogen leaving group, which is estimated to be about 20% per deuterium atom at 80°C. The small isotope effect supports the transition state model involving participation of the electrons localized on the iron atom. The activation parameters for the solvolysis reaction in the temperature range 70–90°C are: ΔH ≠ = 132.6 ± 3.8 kJ mol −1 and ΔS ≠ = 54.0 ± 20.1 JK −1 mol −1 .

Kinetics and mechanism of replacement of sulphite in the pentacyano(sulphito)ferrate(II) ion by cyanide ion

The kinetics of replacement of sulphite by cyanide ion in Na5[Fe(CN)5SO3] in aqueous solution follow the rate law –dln[Fe(CN)5SO35–]/dt=k1k2[CN–]/(k–1[SO32–]+k2[CN–]), which is consistent with reactions (i) and (ii). [Fe(CN)5SO3]5– [graphic omitted] [Fe(CN)5]3–+ SO32–(i), [Fe(CN)5]3–+ CN– [graphic omitted] [Fe(CN)6]4–(ii) At 43 °C, pH 10·80, and 1M ionic strength k1=(9·50 ± 0·21)× 10–4 s–1. The [Fe(CN)5]3– intermediate is sufficiently long-lived to exhibit selective reactivity toward different reagents as shown by the competition factor k2/k–1= 8·76 ± 0·53. The kinetic data are considered as evidence for a limiting SN1 mechanism. The competition factor has been interpreted in terms of free-energy changes resulting from desolvation processes of the competitors.

Anation of penta-ammineaquocobalt(III) by bromide ion in aqueous media

The aquation rate constant of the ion [Co(NH3)5Br]2+ is (3·9 ± 0·08)× 10–6 s–1 at 25·0 °C 1·00M ionic strength, and 0·0100M-HClO4. The anation rate constant for the same conditions is (1·32 ± 0·06)× 10–6 l mol–1 s–1 over the range 0·1000M⩽[Br–]⩽ 0·750M and shows no saturation effect with increasing bromide-ion concentration. The mechanistic implication of this result is discussed. By independent measurements the concentration quotient for formation of the inner-sphere bromo-complex has been found to be 0·283 ± 0·030 l mol–1 for the same conditions. Attempts to determine ion-pairing constants of Br– with [Co(NH3)5H2O]3+ and [Co(NH3)5Br]2+ failed.

Kinetics and mechanism of replacement of nitrosobenzene in the pentacyano(nitrosobenzene)ferrate(II) ion by cyanide ion

The rate of replacement of nitrosobenzene by cyanide ion in the complex [Fe(CN)5(PhNO)]3– increases non-linearly with the cyanide-ion concentration, reaching an asymptotic value at [CN–]ca. 2 × 10–3M in 1·5 × 10–4M complex solution. A limiting SN1 mechanism is proposed.