Debabrata Banerjea

Kinetics and mechanism of dissociation of ethylenedibiguanidenickel(II) and copper(II) and of bisbiguanide-nickel(II) in acid

SummaryThe kinetics of dissociation of the ethylenedibiguanidenickel(II) and copper(II), [M(EndibigH2)]2+, where M = NiII or CuII, and bisbiguanide-nickel(II), [Ni(BigH)2]2+, complexes in acid media forming the aquo-metal ions and the protonated ligand as the ultimate products have been studied by the stopped-flow technique. The reactions occur in two consecutive steps, the first being faster than the second, forming aquometal ions and protonated ligands as the ultimate products. For each step the rate is acid dependent and may be expressed by: kx = kx′ [H+] + kx″ [H+]2, where kx is the observed rate constant and x = “f” or “s” for fast and slow steps respectively. Both paths (kx′ and kx″) contribute in the ethylenedibiguanidenickel(II) system, whereas kx′ paths are virtually absent in the corresponding copper(II) complex; kx″ paths are absent in the bisbiguanidenickel(II) system. A likely mechanism involves protonation of the bound ligand which facilitates its dissociation. A comparison of the ΔS≠ values indicates considerable solvent participation in the transition state, suggesting an assisted dissociative mechanism, which also accounts for the low ΔH≠ values. The observed lability order is [Ni(EndibigH2)]2+ < [Ni(BigH)2]2+ =ca. [Cu(EndibigH2)]2+, and [Cu(BigH)2]2+ dissociates under comparable conditions at a rate too fast to be measured by the stopped-flow method.

MECHANISM OF METAL ION PROMOTED DISSOCIATION OF SOME OXALATO COMPLEXES OF CHROMIUM(III)

Abstract Dissociation of oxalato complexes of chromium (III) of the type [Cr(ox)n (OH2)6–2n](3–2n)+ into [Cr(ox)n-1 (OH2)8–2n](5–2n)+ (where n = 1, 3) promoted by different metal ions of the first transition series in acid perchlorate media has been investigated spectrophotometrically. In comparable cases the order of the catalytic activity as measured by the second-order rate constant kM, follows the sequence Fe3+ >> Cu2+ > Ni2+ > Zn2+ > Co2+ > Mn2+. Log kM for a particular substrate complex and different promoter metal ions of same charge varies linearly with log KM-ox, KM-ox being the formation constant of the mono-oxalato complex of the catalysing metal ion Mn+. The value of kH+ for the acid catalysed reaction occupies a position intermediate between kCo2+ and kMn2+ and even for the positively charged complex [Cr(ox) (OH2)4]+, Fe3+ is a far better catalyst than H+, indicating the importance of specific bonding between the promoter and the substrate complex in the intermediate step. These conclusions a...

Kinetics and mechanism of formation and dissociation of copper(II) and nickel(II) complexes of the Schiff base bis(acetylacetone)ethylenediimine in aqueous-organic solvent media

SummaryIn NH4NO3+NH4OH buffered 10% (v/v) dioxan-water media (pH 7.0–8.5), thePseudo-first-order rate constant for the formation of the title complexes M(baen),i.e. ML, conforms to the equation 1/kobs=1/k+1/(kKo.s · TL), where TL stands for the total ligand concentration in the solution, Ko.s is the equilibrium constant for the formation of an intermediate outer sphere complex and k is the rate constant for the formation of the complex ML from the intermediate. Under the experimental conditions the free ligand (pKa>14) exists virtually exclusively in the undissociated form (baenH2 or LH2) which is present mostly as a keto-amine in the internally hydrogen-bonded state. Although the observed formation-rate ratio kCu/kNi is of the order of 105, as expected for systems having “normal” behaviour, the individual rate constants are very low (at 25°C, kCu=50 s−1 and kNi=4.7×10−4s−1) due to the highly negative ΔS≠ values (−84.2±3.3 JK−1M−1 for CuL and −105.8±4.1 JK−1M−1 for NiL); the much slower rate of formation of the nickel(II) complex is due to higher ΔH≠ value (41.2±1.0 kJM−1 for CuL and 78.2±1.2 kJM−1 for NiL) and more negative ΔS≠ value compared to that of CuL. The Ko.s values are much higher than expected for simple outer-sphere association between [M(H2O)6] and LH2 and may be due to hydrogen bonding interaction.In acid media ([H+], 0.01–0.04 M) these complexes M(baen) dissociate very rapidly into the [M(H2O)6]2+ species and baenH2, followed by a much slower hydrolytic cleavage of the ligand into its components,viz. acetylacetone and ethylenediamine (protonated). For the dissociation of the complexes kobs=k1[H+]+k2[H+]2. The reactions have been studied in 10% (v/v) dioxan-water media and also ethanolwater media of varying ethanol content (10–25% v/v) and the results are in conformity with a solvent-assisted dissociativeinterchange mechanism involving the protonated complexes.

KINETICS AND MECHANISM OF DISSOCIATION OF SOME BIS-(N1-SUBSTITUTED BIGUANIDE) COMPLEXES OF NICKEL(II) IN AQUEOUS ACID

Abstract The rates of dissociation of bis-(N1-methylbiguanide)nickel(II) and bis-(N 1-phenylbiguanide)nickel(II) in aqueous acid have been studies by the stopped-flow spectrophotmetric technique. The reactions occur in two consecutive steps, the first being faster than the second, ultimately forming the aquometal ion and the protonated ligand. For each step the rate is acid dependent as expressed by kx=k1 x[H+], where kx is the observed rate constant. The results are consistent with a mechanism involving an assisted dissociative process involving the protonated complexes with considerable solvent participation in the transition state. The observed lability order is Ni(PhbigH)2+ 2 > Ni(MebigH)2+ 2 > Ni(BigH)2+ 2.

Thermodynamic and Kinetic Studies on the Iron(III)-Hydroxamate Interaction in Acid Media

Abstract Thermodynamics and kinetics of 1:1 complexation of iron(III) with benzo- and several ortho-substituted (CH3, NH2, Cl and OH) benzohydroxamic acids (HL) forming FeL2+ have been investigated spectropho-tometrically in aqueous perchloric acid solution. Under the experimental conditions ([H+] « TFc « THL) complexation involves reactions of HL with Fe3+ (k1 path) and FeOH2+ (k2 path) ions leading to an equilibrium. Both k1 and k2 have been evaluated by stopped-flow spectrophotometry following the formation of FeL2+ and the dissociation of FeL2+ in acid solution. The AH* value (42.5 ± 4.8 Hmol-1) for the k2 path is close to that for the water exchange of FeOH2+ species, suggesting essentially a dissociative (Id) process, but that (54.5 + 3.8 kJmol-1) corresponding to the k1 path is perceptibly lower than that for the water exchange of Fe3+ species indicating associative (Ia) character. The reverse rate constants (k-1 and k_2) for the dissociation of the complexes by the two paths have also been evaluat...

Complexation of uranium(VI) by salicylate and substituted salicylates: A kinetic study

SummaryKinetic studies on the complexation of uranium(VI) by salicylate and various substituted salicylates have been carried out using the stopped-flow spectrophotometric technique at pH 7.0–8.5 (NH4OH+NH4NO3 buffer). Results are in conformity with a mechanism involving binding of UO2OH+ species through the carboxylate group of the salicylate to form an inner-sphere species in a fast equilibrium (equilibrium constant=K) followed by a slow rate-determining ring closure (rate constant=k) involving loss of a molecule of water between the OH group bound to uranium(VI) and the phenolic group of the salicylate. The value of the equilibrium constant (K) obtained from the kinetic data in the case of 5-sulphosalicylate (log K=3.21 at 25 °C, I=1 M) is compatible with the literature thermodynamic value (log K = 3.89 at 25 °C, I=0.015 M). Increase in pH retards the reaction due to the equilibrium, UO2OH+ + OH−⇌ UO2(OH)2, the UO2(OH)2 being unreactive. The average value of K″ (log K″=8.58 at 25°C, I=1M) obtained kinetically from the results of investigation with different ligands is also in good agreement with the literature thermodynamic value (log K″= 8.8 at 25°C, I=0.1M). Both K and k are sensitive to the nature of the substituent in the benzene ring, decreasing with increasing acidity of the -CO2H group of the salicyclic acid; the substituent effect is well demonstrated by the plot of log kversus σL′ (where

Kinetics and mechanism of oxidation of 8-hydroxyquinoline and its derivatives by cerium(IV) through precursor complex formation

\sigma _L^\prime = pK_{CO_2 H}^{salicylic acid} - pK_{CO_2 H}^{substituted salicylic acid}

Kinetic studies on the formation of ternary complexes in the reaction of diaquonitrilotriacetatonickelate(II) with amino acids in aqueous solution

), which is linear. ΔH# and ΔS# values corresponding to k have been evaluated in each case. ΔS# values are all negative in conformity with ring closure in the rate-determining step.