Showing papers on "Aquation published in 1995"

Formation of W/O Microemulsions in the Extraction System Rh(III)-HCl-Kelex 100 and Its Impact on Rh(III) Distribution

Abstract: The extraction behavior of chloro complexes of rhodium (III) into an organic phase of Kelex 100 extractant has been investigated from the standpoint of aquation and microemulsion formation. The aquation of Rh-chloro complexes has been confirmed experimentally to be the main factor which suppresses the liquid-liquid extraction of rhodium. In addition, the formation of Kelex 100-acid-water aggregates in the organic phase has been unequivocally demonstrated by performing various measurements such as water uptake, viscosity, interfacial tension, and light scattering. Due to aquation which is promoted by the formation of microemulsions, very limited overall rhodium extraction can be effected by direct liquid-liquid extraction.

Complex-formation reactions of aquacobalamin revisited: effect of chloride on the rate and activation parameters

Abstract: Complex-formation reactions of aquacobalamin were reinvestigated to clarify the role of high chloride concentrations (up to 1 mol dm–3). It was found that high chloride concentrations retard the reactions due to the formation of the less substitution-labile chloro complex, for which the formation constant was found to be 0.8 dm3 mol–1 at 25 °C and ionic strength 1.0 mol dm–3. The pH dependence of the complex formation with hydrazoic acid was also re-examined. Volumes of activation were determined for the reactions with HN3 and N3– in NaClO4 medium and are discussed in comparison with those measured in KCl medium. Direct kinetic evidence for the occurrence of a reverse, acid-catalysed aquation reaction was found and this step is characterised by an activation volume of +8.3 ± 1.6 cm3 mol–1. The results are all in agreement with the concept of a dissociative interchange substitution mechanism.

Complexes of Rh(C5Me5) with picolinic acid, pyrones and pyridinones

Abstract: Picolinic acid (pyridine-2-carboxylic acid), pyrones and pyridinones (HL) reacted with [{Rh(C5Me5)Cl2}2] and sodium methoxide to give complexes [Rh(C5Me5)Cl(L)] which are all soluble in water. The structures where HL = picolinic acid or 2-methylpyran-4-one have been determined by X-ray diffraction. The aquation of the complexes when dissolved in water has been examined by NMR spectroscopy and conductivity measurements and the results indicate an equilibrium between chloro and aqua species which is fast on the NMR time-scale.

Electrochemistry and spectroelectrochemistry of an α-iminooxime iron(II) macrocyclic complex.

Abstract: Herein we report on the electrochemistry and spectroelectrochemistry of the complex [Fe(imox) (Nmin)2]+, 1, where imox = 2,3,9,10-tetramethyl-1,4,8,11-tetraazaundecane-,3,8,10-tetraen-11-ol-1-olate, and Nmim = N-methyl imidazole. In aqueous solution, 1 undergoes aquation producing [Fe(imox)(Nmim)(H2O)]+, which under one-electron oxidation leads to formation of a μ-hydroxo iron(III) dimer with an estimated Kdim value of 1.0 × 104. The forward and reverse rate constants for the dimerization reaction are k1 = 1.8 × 103 M−1s−1 and k−1 = 0.18−1, respectively.

Dalton communications. Effect of amine substituents and neutral leaving groups on the activation volume for aquation of octahedral pentaamine complexes of CrIII and RhIII

Abstract: The activation volumes were obtained for the aquation reactions of [M(RNH2)5L]3+(M = Cr, R = Me, L = Me2SO, HCONMe2 or MeCONMe2; M = Rh, R = H or Me, L = Me2SO or HCONMe2); they are in accord with the existence of a dissociative shift in the substitution mechanism, not only with the size of the substituent R, but also with the size of the leaving ligands L.

Substitution reactions of trans-[Ru(NH3)4{P(OEt)3}(H2O)]2+ revisited. Mechanistic elucidation from a volume-profile analysis

Abstract: The complex-formation reactions of trans-[Ru(NH3)4{P(OEt)3}(H2O)]2+ with L = imidazole (Him), isonicotinamide (isn) and pyrazine (pyz) were studied in aqueous solution (I= 0.10 mol dm–3, CF3CO2Na) as a function of entering ligand concentration and pressure up to 100 MPa, at 25.0 ± 0.1 °C. The volumes of activation for the complex-formation reactions were +4.2 ± 0.2 (pH 8.6), +1.9 ± 0.3 (pH 5.3) and +2.0 ± 0.3 cm3 mol–1(pH 5.0), for L = Him, isn and pyz, respectively. In the case of isn and pyz as entering ligands the volumes of activation for the reverse aquation reactions were found to be +7.5 ± 0.4 and +10.4 ± 0.3 cm3 mol–1, respectively. Based on the volume of activation data and the constructed volume profiles a dissociative interchange mechanism is proposed.

Reaction of [Fe2(CN)10]4– with L-ascorbic acid

Abstract: The oxidation of L-ascorbic acid by the iron(III) dimeric complex [Fe2(CN)10]4– in acidic aqueous solution at an ionic strength of 1.0 mol dm–3(NaClO4) has been studied by stopped-flow spectrophotometry. The overall reaction, which entails reduction of both iron(III) centres followed by aquation resulting in the formation of 2 mol of [Fe(CN)5(OH2)]3–, takes place in three stages. The first is a one-electron reduction to form the mixed-valence iron(III, II) dimer followed by a second one-electron reduction of the other iron(III) centre to form the iron(II) dimer. The third stage which occurs with a very small absorbance change was not studied. The mechanisms for both stages of the reaction are similar and involve formation of an ion triplet [Fe2(CN)10]n–·M+·Ap– where n= 4 or 5 and Ap– is the reacting ascorbate species with p= 1 or 2. The rate law was found to be of the form (i) where [A]T is the total concentration of added ascorbate, k1 and k2 the acid dissociation constants Rate =k′K1[H+]+k″K1K2//K1K2+K1[H+]+[H+]2[A]T[complex](i), of ascorbic acid and k′ and k″ the pseudo-second-order rate constants. They were found to be 6.44 × 103, 2.48 × 109 and 1.25 × 103, 2.98 × 108 dm3 mol–1 s–1 at 19 °C for HA– and A2– for the first and second stages respectively. The reaction is affected by alkali-metal cations, increasing rate with increasing size of the metal ions: Li+ < Na+ < K+.

Kinetics of dissociation of nickel complexes with pentadentate ligands

Abstract: The synthesis of the pentadentate ligands 1,4-bis(2-pyridylmethyl)-1,4,7-triazacyclononane (BPTAN) and 1,4,7-triazacyclononane-N,N′-diacetate (TCDA) are reported. The nickel(II) complexes of BPTAN and TCDA were synthesized and characterized by their elemental analyses, electrochemistry, and uv-vis spectra. The rate of nickel ion aquation from the BPTAN complex in aqueous solution in the presence of chloride ion was found to follow the rate law, Rate = k1[H+]2[Cl][NiL]/{1 + k2[Cl]}. At 60°C and I = 2.0 M, values for k1 and k2 were determined to be (1.7 ± 0.2) × 10−6 M−3 s−1 and 0.15 ± 0.04 M−1 respectively. For NiTCDA(H2O), the rate law was found to be independent of the chloride ion concentration. The rate law was found to follow the form, Rate = k1[H+][NiL]/(1 + k2H+]). At 60°C and I = 2.0 M, k1 and k2 were found to be equal to 3 M−1 s−1 and 0.90±0.07 M−1, respectively. This work has shown NiBPTAN(H2O)2+ to be one of the most inert nickel complexes to ligand dissociation that has been reported.

Steric Trans Influence of Aminomethane Ligand in trans-Aminomethanechlorobis(1,2-diaminoethane)cobalt(III)

Abstract: The trans isomer of aminomethanechlorobis(1,2-diaminoethane)cobalt(III), trans-[CoCl(NH2Me)(en)2]2+, was prepared from trans-[CoCl(NO2)(en)2]+ by a two-step reaction in which Cl− was replaced with an NH2Me and then the remaining NO2− was replaced with a Cl−. The configuration was confirmed by X-ray crystal analysis, and structural comparison was made with its cis isomer. The crystals have the formulae, trans-[CoCl(NH2Me)(en)2]Cl(ClO4) and cis-[CoCl(NH2Me)(en)2]Cl2. The crystallographic data are as follows; Monoclinic, P21/c, a = 11.515(1), b = 7.421(2), c = 17.257(1) A, β = 99.220(8) °, V = 1455.6(5) A3, Z = 4, and R = 0.043 for the former, and Orthorhombic, Fdd2, a = 26.64(1), b = 27.26(1), c = 7.570(2) A, V = 5497(3) A3, Z = 16, and R = 0.057 for the latter. The Co–Cl bond of the trans complex is longer than that of the cis complex. It was found that the bulkiness of the Me group effects the trans Co–Cl bond through one of the two flexible equatorial en chelate rings. The kinetic data of an aquation rea...

Effects of solvents on the reactions of coordination complexes. Part 20. Kinetics and mechanism of base hydrolysis of (αβS) (Salicylato) (Tetraethylenepentamine) Cobalt(III) in aquo-organic solvent media

Abstract: The kinetics of base hydrolysis of (αβS) (salicylato) (tetraethylenepentamine) cobalt(III) have been investigated in aquo-organic solvent media at 15.0 < t, °C < 40.0, and I = 0.10 mol dm (ClO4−) using propane-2-ol (⩽70% v/v), t-butanol (⩽60% v/v), acetone (⩽70% v/v), acetonitrile (⩽50% v/v), and ethylene glycol (⩽70% v/v) as cosolvents. Both the spontaneous and base-catalyzed hydrolysis of the phenoxide species [(tetren)CoO2CC6H4O]+ were appreciably accelerated by the cosolvents PriOH, ButOH, Me2CO, and MeCN. On the contrary the base hydroylsis (k2) was retarded while spontaneous aquation (k1) was accelerated to a small extent with increased EG content. Variation of log k1 and log k (k = k2 at I = 0) with mole fraction (X0.S) or reciprocal of the relative permitivity (Ds−1) of the media were nonlinear. The transfer free energy of the transition state relative to that of the initial state of the substrate for transfer of species from water to mixed solvents also varied nonlinearly with X0.S, or Ds−1 indicating solvent specificity. The activation parameters, ΔH≠ and ΔS≠ varied nonlinearly with solvent composition exhibiting extrema. The preferential solvation and solvent structural effects mediated the kinetics and energetics of the reaction. © 1995 John Wiley & Sons, Inc.

Aquation of tris(3,4,7,4-tetramethyl-1,10-phenanthroline) iron(II) in aqueous triton X-100 solution

Abstract: The effect of triton X-100 micelle on the aquation of Fe(Me4phen) 3 2+ has been investigated. In acidified (1.0 and 1.5 mol dm−3 H+) aqueous solvent the reaction is inhibited by triton X-100 by factors of 2.4 and 2.7, respectively. The kψ — surfactant profile is not significantly structured due to the low aggregation number of triton X-100 micelle. Inhibition is proposed to be due to favorable hydrophobic interaction with accompanying stabilization of the Fe(II) complex with respect to dissolution.

Solvent/substrate effects on the micelle-catalyzed aquation of some iron(II)-phenanthroline complexes, II. Influence of urea and sodium benzoate on the aquation of Fe(Ph2Phen) 3 2+ in acetone

Abstract: The micelle-catalyzed aquation rates of iron(II) phenanthroline complex Fe(Ph2Phen)32+ have been measured in aqueous acetone as solvent in the presence of urea and sodium benzoate as substrates. The variation patterns manifested by the estimated maximum catalytic factors and kovs [substrate] profiles show that the aquation rates of the complex are inhibited by urea, while they are significantly enhanced by sodium benzoate. These observations have been radionalized by considering the relative micelle bond-breaking capacities of the substrates, the relative strengths of the H-bonds formed by the substrate/OH− headgroups of the micelle and the H2O/OH− headgroups of the micelle pairs.

Ligand‐Substitution Reaction of Cobalt(III) Complexes of [CoCl(N)5]2+‐type with Carbonate: Activation of Kinetic Processes by Steric Chelate Effect

Abstract: Kinetics and mechanisms of coordination by ligand-substitution of HCO 3 - for Cl - on the cobalt(III) complexes of [CoCl(N) 5 ] 2+ -type having different (N) 5 backbone chelate ring structures to form those of [Co(CO 3 )(N) 5 ] + -type have been investigated in a weakly alkaline aqueous 0.10 mol dm -3 sodium perchlorate solution at ambient temperatures, in order to elucidate any possibility for the fixation of atmospheric CO 2 on the coordination sphere of the transition metal center in the form of CO 3 2- . The HCO 3 - in the aqueous solution easily and substitutionally coordinates to the cobalt(III) complexes in place of coordinated chloro ligand via two consecutive rate-determining steps, the faster one with two parallel pathways of aquation and hydroxide-anation and the slower one with a pathway of carbonate-anation. Of these reaction pathways, the hydroxide-anation was explained by a dissociative interchange mechanism via conjugate base (I d CB), whereas the carbonate-anation was explained by a nucleophilic attack of coordinated oxygen atom on the central cobalt(III) atom to the carbon atom of HCO 3 - while maintaining the Co-O bond throughout the step by taking the result of an EHMO calculation into consideration. The (N) 5 backbone chelate ring structure served to enhance the rate of coordination of HCO 3 - on the cobalt(III) center as well as that of preceding aquation and hydroxide-anation steps chiefly by its entropy effect. The pentadentate tetren backbone structure was recommended for the rapid fixation of CO 2 in the form of CO 3 2- on the cobalt(III) complexes of [CoCl(N) 5 ] 2+ -type.