Showing papers on "Aquation published in 1978"

Vibrational spectral studies of ion-ion and ion-solvent interactions. III. Zinc nitrate in water/acetonitrile mixtures

Abstract: Raman and infrared spectral data have been collected forp-dioxane and solvated and bound nitrate ions for Zn(NO3)2/p-dioxane/water systems. It is concluded that Zn2+ is preferentially solvated by water and that this aquation is also responsible for a lower concentration of ion pairs than is found for methanol/Zn(NO3)2 solutions for which the dielectric constant of the bulk solvent is similar. Values of K1 (M−1), the association constant for Zn(NO3)+, are 0.22±0.02 (2/1 solvent,D=12.6) and 0.071±0.01 (4/1 solvent,D=33.0). The log K1 against 1/D plot is not linear.

Kinetics of reaction of imidazole, glycine, and L-histidine with the aqua-pentacyanoferrate(II) ion

Abstract: The properties and reactivity of the pentacyanoferrate(II) complexes of imidazole (imH), glycinate (glyO), and L-histidine (his) are reported. The rates of formation, starting from [Fe(CN)5(OH2)]3–{generated by aquation of [Fe(CN)5(NH3)]3–}, are typically of first order with respect to the concentration of the ligands, with kf= 240, 28.0, and 320 dm3 mol–1 s–1, respectively. The kinetics of dissociation of the complexes, measured in the presence of an excess of dimethyl sulphoxide, show saturation behaviour with respect to this reactant. The limiting rates of dis-sociation are 1.33 × 10–3(imH), 2.67 × 10–3(glyO–), and 5.3 × 10–4 s–1(his) at 25 °C and I= 0.100 mol dm–3(Li[ClO4]). L-Histidine, in zwitterionic form, reacts with [Fe(CN)5(OH2)]3– to yield two isomers in solution. The predominant form contains iron co-ordinated to the hindered N3 nitrogen of imidazole in the L-histidine ligand. This isomer is labile and dissociates rapidly, k= 0.109 s–1 to form the inert and stable N1 isomer. Similar behaviour, but involving the three N-co-ordinating sites of histidine, is observed in alkaline solutions. The strain energy associated with substitution adjacent to the iron centre is estimated as 3,4 kcal mol–1 A correlation of the limiting rates of dissociation with the ligand-field energies in the complexes is presented.

Kinetics and mechanism of aquation and formation reactions of carbonato complexes. 12. Deuterium solvent isotope effect on the rate of acid-catalyzed decarboxylation of the carbonatobis(ethylenediamine)cobalt(III) complex ion. A mechanistic reappraisal

Abstract: A recent study of the acid-catalyzed decarboxylation of the carbonatotetrakis(pyridine)cobalt(III) complex ion showed there to be rate acceleration in D/sub 2/O solvent, consistent with a proton-preequilibration mechanism. This observation directly contradicts the results of a similar study made some years ago of the analogous ion, carbonatobis(ethylenediamine)cobalt(III), for which there appeared to be deceleration in D/sub 2/O solvent. A reinvestigation of the latter reaction over a much wider acidity range has now shown the earlier work to be in error. The previously proposed generalized mechanism for aquation of chelated carbonato complex ions of the form CoN/sub 4/CO/sub 3//sup +/ (N/sub 4/ identical with various tetramine ligand groupings of uni-, bi-, or quadridentate type) has thus been revised to include a proton equilibration step. An unexpected complication arises in the interpretation of the data for the bis(ethylenediamine) complex ion in the acidity range 0.1 < (H/sup +/) < 1.0. Within these limits, the pseudo-first-order rate constant for carbonato chelate ring opening, which includes a (H/sup +/) term, overtakes and exceeds the true first-order rate constant for CO/sub 2/ release. The interesting implications of this unusual first-order successive reaction system are fully explored in the context of the present study.

Influence of Ligand-Water Interactions on the Aquation of Pentacyano(saturated amine)ferrate(II) Ions

Abstract: Rate constants and activation parameters for the aquation of 12 saturated amines from pentacyanoamineferrate(I1) in 1 M pyridine solutions are reported. While AG* values do not correlate with AGIO (for the ionization of the conjugate acid) or with u* and E, Taft's constants, an isokinetic relationship is found. For the series (CH3)xSH3_, and RSH2, AP correlates with A",' in water and each series shows an independent correlation of AS* with B,". These results are interpreted in terms of an adapted Caldin and Benetto's model for exchange reactions, the main contribution to the changes in activation parameters arising seemingly from the energetics of transfer of the released ligand to bulk water. This small contribution to the activation process in water is rendered observable by the remarkable insensitivity of the rate process to "inner" bonding effects.

Hydroxide ion assisted aquation of tris(2,2'-bipyridine)chromium(III) ion

Abstract: E,, kcal/mol 22.9 i 0.2 21.3 i 0.2 19.5 i 0.5 AH*296, kcal/mol 22.3 f 0.2 20.7 I: 0.2 18.9 i 0.5 InA 26.1 i. 0.1 24.1 i 0.2 24.8 r 0.9 AG 296, kcal/mol 24.9 i. 0.3 24.5 i 0.3 22.2 i 1.1 k,,,, s-' 3.3 x 6.9 x 2.9 x

The trans effect in cobalt(III) complexes. Kinetics and mechanism of substitution of cobalt(III) sulphito complexes

Abstract: Equilibrium constants K and rate constants kf have been measured, at 25°C and ionic strength of 1.0, for the substitution of the labile water molecule in trans-[aquabis(ethylenediamine)sulphito-cobalt(III)] ion by thiosulphate ion (K = 1.8×102 mol-1 1., kf = 1.27×103 mol-1 1. s-1), thiocyanate ion (2.5×103, 2.75×102), nitrite ion (1.0×103, 2.06×102), azide ion (2.9×102, 2.4×102) ferricyanide ion (-, 1.72×103), hydrogen azide (< 1.2,1.4×10), ammonia (3.0, 6.7) and imidazole (2.6×102, 5.2). The correlation of these rate constants with charge on the incoming ligand, as well as a decrease in the apparent second-order rate constants observed at high concentrations of the anionic ligands, requires a rapid outer-sphere pre-equilibrium step followed by a rate- determining dissociative interchange of the incoming ligand with the bound water molecule. The activation energy of the thiocyanate substitution was found to be 48 kJ mol-1. Aquation of cis- [azidobis(ethylenediamine)-sulphitocobalt(III)] ion, in the range of hydrogen ion concentration between 10-2 and 0.2 M, was found to give the trans-aquasulphito complex with a first-order rate constant consistent with the equation k = 4.9×10-4[H+]+1.0×10-5 s-1 at 25°C and ionic strength 1.0.

Hydroxide ion assisted aquation of tris(2,2'-bipyridine)chromium(III) ion

Abstract: E,, kcal/mol 22.9 i 0.2 21.3 i 0.2 19.5 i 0.5 AH*296, kcal/mol 22.3 f 0.2 20.7 I: 0.2 18.9 i 0.5 InA 26.1 i. 0.1 24.1 i 0.2 24.8 r 0.9 AG 296, kcal/mol 24.9 i. 0.3 24.5 i 0.3 22.2 i 1.1 k,,,, s-' 3.3 x 6.9 x 2.9 x

Kinetics and mechanism of aquation and formation reactions of cis-carbonatobis(oxalato)chromate(III) ion in aqueous solution

Abstract: The title compound has been prepared and characterized for the first time. The kinetics of the aquation and formation reactions of this carbonato complex were studied by stopped-flow spectrophotometry. The acid catalysis reactions were studied in the range of 2 > [H’] > 0.01 M and 10 < T < 25 O C at an ionic strength of 2 M (NaC10,). Only the ring-opening reaction of Cr(C0,)(0x),~is observed, k = 2.04 M-’ s-’ at 25 OC, as subsequent decarboxylation is too fast to measure even at 2 M hydrogen ion concentration. Carbon dioxide uptake by cis-diaquobis(oxalato)chromate(III) was investigated over the ranges 7.01 < pH < 9.34 and 10 < T < 25 O C at an ionic strength of 0.5 M (NaCI). The major reactant species in the pH range studied was ~ i s C r ( o x ) ~ ( O H ) ( 0 H ~ ) ~ which takes up C02 to form ~is-Cr(ox)~(HC0~)(0H~)~. Subsequently, slower ring closure of the latter species to form the carbonato chelate is observed. The results are compared to those of cobalt complexes previously studied and the possible mechanisms are discussed.

Kinetics of reactions of Schiff-base complexes of iron(II). Part 6. The preparation and kinetics of reactions of complexes of multidentate ligands

Abstract: Preparations of iron(II) complexes of bi-, tri-, quadri-, quinque-, and sexi-dentate Schiff bases are reported. The Schiff-base ligands are derived from pyridine-2-carbaldehyde, phenyl 2-pyridyl ketone, pyridine-2,6-dicarb-aldehyde. or 2,6-diacetylpyridine, and appropriate amines. All these complexes aquate in acid solution, and react with cyanide to give ternary iron(II)–cyanide–Schiff-base complexes. The kinetics of aquation of several of these Schiff-base complexes, and the kinetics of cyanide attack at all the complexes, are described, and reactivities are compared with those for analogous reactions of other iron(II) di-imine complexes. Solvent effects on the re-activity with cyanide of one of the complexes of the sexidentate Schiff bases in a range of binary aqueous mixtures are discussed.

Volumes of activation for racemization and aquation of tris(1,10-phenanthroline) complexes of nickel(II) and iron(II) in solution

Abstract: The effect of high pressure on the rates of racemization and aquation of Ni(~hen)~~+ and Fe(phen)32+ (phen = 1,lOphenanthroline) in solution has been evaluated. Reactions of Fe(~hen)~'+ exhibit a significant retardation upon the application of pressure, with volumes of activation (AV) of +14.2 and +15.6 cm3 mol-' for racemization in 0.01 and 1.0 M aqueous HC1, respectively, while aquation in 1.0 M aqueous HC1 exhibits a AV of +15.4 cm3 mol-'. Possible mechanisms are considered, and a positive contribution to AP from excitation to a high-spin transition state, associated with a general Fe-N bond expansion, is proposed as a component of the mechanism for aquation and racemization. Reactions of Ni(phen)p show only a minor acceleration of rates upon the application of pressure with AV = -0.4 cm3 mol-' and AV = -1.5 cm3 mol-' for racemization in 0.01 and 1.0 M aqueous HC1, respectively, while aquation in 1.0 M aqueous HCI exhibits a AV of -1.2 cm3 mol-'. A common mechanistic pathway is favored since determined rates and enthalpies and entropies of activation as well as volumes of activation for aquation and racemization are very similar. A dissociative mechanism for release of a phen ligand with possible formation in the transition state of a high-spin four-coordinate species of reduced volume compared with the low-spin octahedral precursor is favored from analysis with AV data.

Kinetische und Gleichgewichtsuntersuchungen zur Aquotisierung und Halogenid-Anation an Halogeno-Aquo-Komplexen von Osmium(IV)

Abstract: Die Komplexe des Typs [OsJ5X]2− (X = Cl, Br, J) unterliegen in saurer, wasriger Losung schon bei Zimmertemperatur der Hydrolyse nach [OsJ5X]2− + H2O ⇌ [OsJ5(H2O)]− + X−. Die Geschwindigkeitskonstanten der Hin-(Aquotisierung) und Ruckreaktion (Anation) sowie die Gleichgewichtskonstanten werden durch spektrophotometrische Messungen bestimmt. Die aus kinetischen und Gleichgewichtsuntersuchungen abgeleiteten thermodynamischen Daten stimmen gut uberein. In der Reihe J, Br, Cl nimmt die kinetische Stabilitat zu, die thermodynamische Stabilitat jedoch ab. Der Monaquokomplex ist um etwa 2 kcal/mol weniger stabil als die Hexahalogenokomplexe. Beim Halogenaustausch last er sich in verdunnten Losungen als Zwischenprodukt nachweisen. Kinetic and Equilibrium Studies on Aquation and Halide-Anation of Osmium(IV)- Halogeno-Aquo Complexes Complexes of type [OsI5X]2− (X = Cl, Br, I) undergo hydrolysis in acidic, aqueous solutions already at room temperature according to [OsI5X]2− + H2O ⇌ [OsI5(H2O)]− + X−. Rate constants of aquation and anation reactions as well as equilibrium constants are determined by spectrophotometrical measurements. The kinetic and equilibrium experimental results are giving similar thermodynamic data. Kinetic stability increases from I over Br to Cl whereas the thermodynamic stability decreases in the same order. The monoaquo complex is less stable than the hexahalo complexes by 2 kcal/mol, and is present as an intermediate product in the halogen exchange reactions carried out in dilute solutions.

Trimethylenediamine Complexes. IV. Kinetics and Mechanism of Acid and Base Hydrolysis of Tetranitro(ethylenediamine)- and Tetranitro(trimethylenediamine)cobaltates(III)

Abstract: Hydrolysis of Co(NO2)4(AA)− (AA=en or tn) was studied in acidic and basic aqueous solutions by the spectrophotometric method. In faintly acidic solution (6>pH>4) the aquation proceeds reversibly to attain an equilibrium with Co(NO2)3(H2O) (AA), Keq being (1.71±0.03)×10−2 and (3.55±0.06)×10−2 mol·1−1 for AA=en and tn, respectively, at 25.0 °C and μ=0.1. With increasing concentration of hydrogen ion the acid-catalyzed pathway becomes increasingly important. In basic solution (12.2>pH>9.5) Co(NO2)3(OH)(AA)− was formed irreversibly as the first product. Kinetic data of these hydrolysis reactions are interpreted in terms of the dissociative mechanism involving a five-coordinate intermediate Co(NO2)3(AA). The rates of nitrite release from Co(NO2)4(tn)− ((9.27±0.03)×10−4s−1) and deaquation from Co(NO2)3(H2O)(tn) ((3.95±0.12)×10−4 s−1) are several times greater than those for the corresponding ethylenediamine complexes ((2.08±0.01)×10−4 and (5.46±0.10)×10−5 s−1, respectively.

On the reaction of oxalate ion withtris-3-(2-Pyridyl)-5, 6-bis-(4-sulphophenyl)-1,2,4-triazine iron (11) in aqueous solution: Kinetics of dissociation of low-spin iron(II) complexes in the presence of polycarboxylate ions

Abstract: Kinetic results for reaction of the title complex with oxalate ion have been reinterpreted in terms of a simple dissociative mechanism in which loss of ligand is rate determining. Competition between the ligand and oxalate ion for the dissociated reactive intermediate causes the observed rate constant to rise to a limiting value with increasing oxalate ion concentration. This rate limit is equal to the rate of dissociation. In agreement with this mechanism, rate constants for reaction of four other related iron(II) complexes with an excess of a range of poly carboxylates were found to be similar to their respective rates of aquation, so demonstrating that polycarboxylares act as scavengers rather than nucleophiles in these reactions.