Showing papers on "Aquation published in 1990"

Mechanistic information from a volume profile treatment for the complexation of aquocobalamin (vitamin B12a) by pyridine

Abstract: The complexation kinetics of aquocobalamin (vitamin B 12a ) by pyridine was studied as a function of pyridine concentration, temperature, and pressure. This kinetic data equally well fit D and I d mechanisms. The pressure dependence of the rate and equilibrium constants enables the construction of an overall reaction volume profile, from which it is concluded that both the forward complex formation and reverse aquation reactions proceed according to a limiting D mechanism

Polyazamacrocycles. Part 7. Kinetics of aquation of trans-dichloro and trans-chloro (nitro) complexes of cobalt (III) with tetra-aza macrocycles

Abstract: Kinetic studies on the aquation of trans-[CoLCl2]+ and trans-[CoL(NO2)+](L = tetra-aza macrocycle) complexes show large variation in rates. The dependence of aquation rates on conformational strain energy is true only for a limited number of compounds. The rate enhancements can be rationalised in terms of the ability of the macrocycle to fold across a fold axis. The large-ringed macrocyclic complexes trans-[CoL6Cl2]+(L6= 1,4,8,11 -tetra-azacyclohexadecane) and trans-[CoL7Cl2]+(L7= 1,4,8,11 -tetra-azacycloheptadecane) aquate at much lower rates due to curtailed folding ability. The N ⋯ N bite distances also suggest a lower aquation rate for these complexes. The complexes of 2,5,5,7,9,12,12,14-octamethyl- and 5,7,12,14-tetraethyl-5,12-dimethyl-1,4,8,11 -tetra-azacyclotetradecane aquate with low rates lying in a narrow range. No specific trends can be identified in these complexes. These studies also show that very large aquation rate enhancements in cobalt (III) complexes of 14-membered macrocycles are possible only if gem-dialkyl groups are present.

Micellar effects upon the reactions of complex ions in solution. Part 4.—Kinetics of aquation and base hydrolysis of some cis-(chloro)(amine)bis(ethylenediamine)cobalt(III) complexes in the presence of neutral and anionic surfactants in an aqueous medium

Abstract: The binding of the substrates cis-[Co(en)2BCl]2+(en = 1,2-diaminoethane, B = alkylamines, imidazole, N-methylimidazole) to the micellar surface of sodium dodecyl sulphate resulted in the retardation of their dissociative aquation rates, the effect being sensitive to the hydrophobicity of the nonlabile amine ligand B. A contrastingly small rate acceleration for the corresponding ethanolamine and propan-2-ol amine complexes was observed under similar conditions. Triton X-100 (0.02 ⩽[Triton X]T/mol dm–3⩽ 0.1) had virtually no effect on the aquation rates of such complexes except for cis-[Co(en)2(C6H11NH2)Cl]2+, in which case a small rate retardation was also observed. The rates of base hydrolysis of the cobalt(III) substrates were strongly retarded by the anionic micelles of SDS; the neutral micelles of Triton X-100 were effective in decelerating the rate of base hydrolysis of the cyclohexylamine complex cis-[Co(en)2(C6H11NH2)Cl]2+ only. The pseudo-phase ion-exchange equilibrium model satisfactorily explained the binding of the cationic substrates to the anionic micellar pseudo-phase of SDS. The values of the ion-exchange equilibrium constant and the relative base hydrolysis rates (kW/kM) indicated that both micellar binding and retardation of hydrolysis are governed by hydrophobic and electrostatic interactions.

Kinetics and mechanism of aquation of cobalt(II)-glycine complexes in aqueous solutions: a pulse radiolytic study

Abstract: The kinetics of aquation of tris(glycinato)cobaltate(II), (Co(gly){sub 3}){sup {minus}}, formed by the reaction of the corresponding Co(III) complex with hydrated electrons, has been investigated by conductometric pulse radiolysis in aqueous solutions. The rate of the reactions leading to the loss of each of the three glycinate ions in (Co(gly){sub 3}){sup {minus}} were well separated from each other and were studied as a function of pH. The observed rate constants are given by k{sup 0} + k{sup H}(H{sup +}). For the glycinate ion dissociation from the Co(II) complexes with one, two, and three glycinate ions, k{sup 0} were 4.9 {times} 10, 3.5 {times} 10{sup 2}, and 4.2 {times} 10{sup 3} s{sup {minus}1}, respectively and k{sup H} were 2.1 {times} 10{sup 4}, 8.1 {times} 10{sup 5}, and 2.7 {times} 10{sup 7} M{sup {minus}1} s{sup {minus}1}, respectively. The kinetics and mechanism of the aquation reaction of (Co(gly){sub 3}){sup {minus}} are discussed and compared to those for the aquation of (Co(en){sub 3}){sup 2+} and (Co(acac){sub 3}){sup {minus}}. 26 refs., 4 figs., 1 tab.

Preparation, crystal structure and kinetics of acid hydrolysis of an isomer of [Cr(picdien)Cl][ZnCl4][picdien = 1,9-bis(2′-pyridyl)-2,5,8-triazanonane]

Abstract: The complex [Cr(picdien)Cl][ZnCl4][picdien = 1,9-bis(2′-pyridyl)-2,5,8-triazanonane] has been prepared and its structure determined by a single-crystal X-ray diffraction study of the triclinic crystals: a= 9.118(3), b= 9.581(5), c= 14.042(5)A, α= 78.04(3), β= 86.37(3)°, and γ= 76.56(3)°. The complex cation has the anti-αβ configuration. The kinetics of aquation of the perchlorate salt has been studied in dilute aqueous nitric acid solution, [complex]= 5 × 10–3 mol dm–3. The rate constants are independent of [H+] over the range 0.10 < [H +] < 0.001 mol dm–3; k25°C= 6.0 × l0–6s–1, ΔH‡= 93.1 ± 2.2 kJ mol–1, ΔS‡=–32.9 ± 6.9 J K–1 mol–1.

Effect of solvent on the reactions of coordination complexes: Part 15: Base hydrolysis ofCis‐(Chloro)(benzimidazole)‐Bis‐(ethylenediamine)cobalt(III) ion in mixed solvent media

Abstract: The kinetics of base hydrolysis ofthecis-(chloro)-(benzimidazole)bis(ethylenediamine) cobalt(III) ion was investigated in Methanol-water and Ethyleneglycol-water media of varying compositions and at isodielectric conditions(D s □ 61 at 25°C) in isopropanol- and Acetonitrile-water media at 35°C in the pH range, 7.2–8.9. It is of interesttonote that the aquation rate constant of the conjugate base, cis-[Co(en) 2 (bzm)Cl] + (Bzm = N-H deprotonated benzimidazole, en = ethylenediamine) is virtually insensitive to the compositions of the mixed solvents studied, unlike the aquation rate constant of the conjugate acid form of the title complex. This is indicative of a true dissociative mode of activation (Dcb) of the conjugate base.

Kinetics and Mechanism of Acid Hydrolysis of Bis(dicarboxylato)carbonatochromate(III) Complexes

Abstract: A study has been made of the acid-catalysed aquation reactions of three complex ions of the general formula [Cr(XX)2(CO3)]3−, where XX=oxalate, malonate, or methylmalonate, by stopped-flow spectrophotometry in the acid range 0.1–2.0 M (M=mol dm−3) and 20–35°C at an ionic strength of 2 M. Only the ring-opening reaction of the carbonate is observed, k=2.12, 3.28, and 4.02 M−1 s−1 at 25°C respectively for oxalato, malonato, and methylmalonato complexes as the subsequent decarboxylation is too fast to measure. The observed trend was explained in terms of the basicity of the coordinated inert ligands.

Ion pairing effect on the aquation knetics of [Co(Py)4Cl2]X (X=Cl?, NO 3 ? , IO 4 ? , ClO 4 ? SCN? and N 3 ? ) in different solvent systems; basicity measurement of the counter ion

Abstract: The aquation kinetics of [Co(Py)4Cl2]X salts (X=Cl−, NO 3 − , IO 4 − , ClO 4 − , SCN− and N 3 − ) in EtOH/H2O have been widely investigated over a range of solvent compositions. Similarly, the aquation rate constants were determined in aqueous solution (1∶1 v/v) containing other organic solvents. The study reveals the existence of specine solvation of the activated complex and that the rate constant associated with the rate-determining step of the reaction depends to some extent on the ionizing power of the medium. The influence of the counter ions on the aquation rate is also discussed. Basicity, the capacity of the nucleophiles to abstract the exchangeable proton from a titanium selenite inorganic exchanger was found to follow the order: SCN−

Kinetics and mechanism of the reaction betweencis-dichlorobisbipyridineruthenium(II) and nitric acid in water-methanol solutions

Abstract: The kinetics of the reaction betweencis-dichlorobisbipyridineruthenium(II) and nitric acid have been investigated spectrophotometrically in the 25°–40° range in the presence of 0.03 to 0.2 mol dm−3 HNO3. The reaction proceeds with the stepwise formation of monoaqua and diaqua products. Only the formation of the monoaqua intermediate was followed as this species could not be obtained in a pure state. Aquation proceeds through a dissociative process. The second order rate constants are 11.8 (25°), 17.5 (30°); 30.0 (35°) l mol−1 s−1. Activation parameters are ΔH‡ 52±3 kJ mol−1; ΔS‡−108±8 JK−1 mol−1.

Ligand substitution in a transition metal complex: a new type of photocatalytic reaction in a semiconductor

Abstract: We show that a semiconductor can serve as a photocatalyst for the ligand substitution reaction in coordination compounds of chromium(III). The mechanism of the process includes stages of generation of an “electron-hole” pair upon action of light quanta with energy equal to (or greater than) the width of the forbidden gap in the semiconductor, capture of the electron from the conduction band by the chromium(III) complex, fast ligand substitution in the chromium(II) complex formed by a solvent molecule, and oxidation of the chromium(II) complex by the hole from the valence band. The reaction of aquation of the complex ions [CrCl2(H2O)4]+ proceeds when using the semiconductors (CdS, GaAs), whose conduction band is located in the more negative potential region compared with the standard oxidation-reduction potential of the Cr(III)/Cr(II) pair. We show that the reaction rate is slowed down when electron acceptors (oxygen) and hole acceptors (ethanol) are introduced, which confirms the proposed mechanism.

Synthesis, characterisation and acid and iron(III) catalysed aquation of trans-bis(Hmalonato)bis(trimethylenediamine) cobalt(III) ion

Abstract: Trans-bis(malH)bis(tmd)cobalt(III) (malH2=HO2CCH2CO2H; tmd=trimethylenediamine) has been synthesised and its acid and FeIII catalysed aquation have been investigated in the temperature range 30–45°C. The major product in both cases iscis-[Co(tmd)2(malH)-OH2]2. The rate laws of the acid and FeIII catalysed aquations are given by $$ - d ln[complex]_\tau /dt = k_0 + k_1 [H^ + ]$$ and $$ - d ln[complex]_\tau $$ $$ = \frac{{k_0 + k_1 [H^ + ] + k_M K_M [Fe^{III} ]/[H^ + ]}}{{1 + K_M [Fe^{III} ]/[H^ + ]}}$$ in the range of [H+]T=0.025–0.5, I=0.5 mol dm−3 and [FeIII]T=0.01–0.06, I=0.8 mol dm−3, respectively. The rates and activation parameters of k0, k1 and kM paths are reported. The rates of both catalysed aquations are found to beca. 200 times faster than those for the corresponding ethylenediamine complex.

The Aquation of Co(III) Complexes in Pure Water and Mixed Water-Organic Solvents With Hg$^{2+}$

Abstract: Observed rate constants (k) for Hg-catalysed aquation of cis-[Co(en)(OH)Cl], [Co(NH)Cl], cis-[Co(NH)(OH)Cl], and [Co(NHCH)Cl] were measured at various concentrations of Hg. The k was increased with increasing the concentrations of Hg. The k were related to mechanistically derived rate constants by the relationship; Rate = kK[complex][Hg]. Various mixed aqueous-organic solvent have been successfully employed for Hg-catalysed aquation of octahedral myetal complexes. From the slope of the plot of log k againt Y (solvent ionizing power), the mechanism on the aquation of Co(Ⅲ) complexes by Hg has been suggested to be I-mechanism. The change in a rate on aquation of each Co(Ⅲ) complex was related to the ligand field parameters (), for Co(Ⅲ) complexes.