Band 5, Heft 1

The kinetics of the title reaction have been investigated in both aqueous and micellar cetyltrimethylammonium bromide(CTAB) solutions. Rate data indicate that the reaction follows the template mechanism in both the media. The observed rate constant is affected by [CTAB] changes and the maximum rate enhancement is ca. 4 -fold. The following parameters have been calculated at pH 5.0: for aqueous, k obs = 5.6 χ 10 -5 s -1 , ΔH* = 61 k J mol -1 , ΔS* = -122 J K -1 mol -1 ; for CTAB , k Ψ = 24.7 χ 10 -5 s -1 , ΔH* = 35 k J mol -1 , ΔS* = -200 J K -1 mol -1 , which reveal CTAB behaving as a catalyst. To explain the dependence of the reaction rate on [CTAB], pseudo-phase model proposed by Menger and Portnoy and modified by Bunton and Rodenas was used. The catalytic role of CTAB can be related to the extent of incorporation or association of the chromium(III)-tryptophan and ninhydrin into the micelles. Various factors which affect k m , K s K N and are discussed taking into account the micellar surface solvent properties.

Piperidine (pip) displaces l,2-bis(diphenylphosphino)ethane (dppe) from mer-(η 2 -C 60 )(η2-dppe)W(CO) 3 to produce mer*-(η 2 -C 60 )(η 1 -ρίρ) 2 \ν(ΟΟ) 3 . The reactions are first order and second order with respect to the molar concentrations of m£r-(r|2-C60)(r|2-dppe)W(CO)3 and pip, respectively. The proposed mechanism, based on the rate law and on the activation parameters, involves an initial pip-assisted ringopening of the dppe ligand to produce the electronically-saturated intermediate fac (η 2 -C 60 )(η'-ρίρ) (n 1 -dppe)W(CO) 3 . This mechanism differs from previously proposed mechanisms for the ligand exchange reactions of closely-related complexes such as/fac-(η 1 -L)(η 2 -dppe)W(CO)3 (L = CO, piperidine, pyridine) and is contrary to the known cis labilizing property of dppe. These results suggest that [60]fullerene has a strong steric influence on the inorganic moiety of the complex and on its chemical behavior.

Using a range of substituted 2,2':6',2"-terpyridine ligands, the effects of π-stacking on the rates of formation of bis(terpyridine)nickel(II) complexes have been investigated in 70:30 v/v % methanol:water. Compared with the rates of formation of the mono(terpyridine)nickel(II) complexes, for the bis(terpyridine)nickel(II) complexes at 25°C, rate enhancements in the range 11-146 were found. For unsubstituted 2,2':6',2"-terpyridine studied in the same solvent mixture, the rate enhancement at 25°C is 11, whereas for 4'-(2-naphthyl)-2,2':6',2"-terpyridine the rate enhancement is 146.

The reaction between Pd(N,N')Cl 2 [(N,N'= l-methyl-2-(arylazo)imidazole (RaaiMe), p-RC 6 H 4 - N=N-C 3 H 2 NN-1 -Me; 2-(arylazo)pyridine (Raap), p-C 6 H 4 -N=N-C 5 H 4 N; 2- (arylazo)pyrimidine (Raapm), p-C 6 H 4 -N=N-C 4 H 3 N 2 ; where R=H (a), Me (b), CI (c)] and apicolinic acid (α-picH) have been examined spectrophotometrically in MeCN solution at 298K. The product is Pd(a-pic) 2 which has been supported by pure compound synthesis from Na2[PdCl4] and α-picH. The kinetics of the nucleophilic substitution reaction has been studied under pseudo-first-order condition. The reaction proceeds in a biphasic manner. The kinetic analysis confirms the nucleophilic association path. External addition of CI - (LiCl) suppresses the rate. On considering the consecutive reaction A-> B-> C the rate data have been evaluated. Each phase shows first-order dependence on [α-picH]. Rate-1 = {a 1 +k 1 [(α-picH]}[ Pd(N,N')Cl 2 ]; rate-2 = {a 1 +k 2 [(α-picH]}[Pd(N,N')Cl 2 ], with the k 1 (first phase rate constant) > k 2 (second phase rate constant). Increase in π-acidity of the ligand N,N' increases the rate and follows the order: Pd(RaaiMe)Cl 2 < Pd(Raap)Cl 2 < Pd(Raapm)Cl 2 .

Kinetic what-if simulations can be important for both the optimisation of an industrial process as well as the design of future experiments in the evolving process of a detailed kinetic analysis on laboratory scale. From our viewpoint as coordination chemists we present SIMKIN, a tool for an easy intuitive and fast simulation of chemical kinetics. As one of the key features, an intelligent model parser translates conventional chemical equations consisting of virtually any reaction steps including feedbacks into the rate law. The corresponding differential equations are then solved by standard routines for numerical integration, and the concentration profiles of the involved species plotted versus the reaction time. By means of selected kinetic examples of increasing complexity taken from coordination chemistry we demonstrate the capabilities and versatility of the program. A MATLAB® version with a complete graphical user interface can be requested from the authors free of charge.

The kinetics of the reaction of chromium(VI) with 2-mercaptoethanesulfonic acid (MESA) has been studied spectrophotometrically at wavelength, 435 nm, and ionic strength, I, of 0.50 mol dm -3 (NaClO 4 ) over the ranges: 16.0 < θ < 29.5 °C, 2.6 < pH < 7.1 (citric acid-phosphate buffer), [Cr(VI)] T = 2.0 χ 10 -4 mol dm -3 , 1.0 χ 10 -2 < [MESA] T < 4.0 χ 10 -2 mol dm -3 . The reaction occurs giving clear indication of the formation of an intermediate, which is assumed to be a typical Cr(VI)-thioester. The decay of the thioester then occurs via a series of electron transfer reactions to give Cr(III). The formation and decay rates of this intermediate have been investigated kinetically and a detailed mechanism for the overall reaction proposed. Experimental data were fitted to rate equations derived from this mechanism. The specific rate constants and corresponding activation parameters were also deduced. The results show that MESA without carboxylate groups, react differently when compared to other thiols having such groups present. This confirms the participating role of carboxylate groups in the formation of the Cr(VI)-thioester. The catalytic effect of added Zn 2+ was investigated and its overall role in the reaction of Cr(VI) with MESA was explained.

Rate constants are reported for ligand replacement in three pentacyanoferrate(III) complexes [Fe(CN) 5 L] 3- with L = 4-cyanopyridine, 4,4'-bipyridyl, or 4-t-butyl-pyridine, in a range of aqueous salt solutions. These salts include chlorides and bromides of potassium, lithium, and calcium, and tetra-alkyl-ammonium bromides, at concentrations within the range 0.50 and 2.00 mol dm -3 . The observed trends are compared with those for a selection of other inorganic reactions.

The kinetics of the interaction of glutathione (reduced) (GSH) with [Pt(en)(H 2 O) 2 ](ClO 4 ) 2 and [Pt(dmen)(H 2 O) 2 ](ClO 4 ) 2 (en = ethylenediamine, dmen = N,N'-dimethylethylenediamine) have been studied spectrophotometrically as a function of [substrate complex], [glutathione] and temperature at a particular pH (4.0). The reaction was found to proceed via rapid outersphere association complex formation followed by two slow consecutive steps. The first step involves the transformation of the outersphere complex into the innersphere complex containing Pt-S bond, while the second step involves chelation when the second aqua ligand is displaced. The association equlibrium constant (K E ) and the two rate constants k 1 and k 2 have been evaluated. Activation parameters for both the steps have been calculated using Eyring equation. The low enthalpy of activation and large negative values of entropy of activation indicate an associative mode of activation for both steps.

A trinuclear complex, [{(NH 3 )5Co(O 2 CCH 2 NH)} 2 Cu](ClO 4 ) 6 .2H 2 O has been synthesised using mononuclear glycinato(pentaammine)cobalt(III) Perchlorate and characterised by various physicochemical methods. The kinetic study by stopped-flow spectrophotometer indicated that the dissociation of the trinuclear complex occurred mainly through an acid catalysed path.