Band 1, Heft 2

The stability constant of the complex formed between cobalt(II) and trimethyltren (Me 3 tren), 2,2′,2″-triaminotriethylamine monomethylated on each terminal amino group, in aqueous medium was determined (log Κ = 12.10 ± 0.01 at 25.0° C and ionic strength of 1.0 mol dm -3 (NaClO 4 )). The deprotonation constant of the aqua complex, [Co(Me 3 tren)(H 2 O)] 2+ , was also established (pK a = 9.31 ± 0.02 under the same conditions). It is concluded on the basis of these values and properties such as the UV/vis spectrum, that the complex possesses trigonal bipyramidal geometry, as does the corresponding copper(II) complex. Dissociation of both complexes, as a function of aqueous acid concentration, was monitored spectrophotometrically. For the copper(II) system there is a modest dependence on the acid concentration when [H + ] < 0.4 mol dm -3 , and above this value the reaction is independent of acid concentration, with the firstorder rate constant reaching a limiting value of about 100 s -1 at 25.0°C. Dissociation of the cobalt(II) complex does not depend on the acid concentration and occurs at a rate of 34 s -1 . A mechanism accounting for these findings is proposed, and the rate and activation parameters are discussed in terms of the dissociation of other trigonal bipyramidal complexes.

The oxidation kinetics for the reaction of [Fe II (1,2- pdta)] (1,2-pdta = 1,2-N,N′-1,2-propylenediaminetetraacetate) and [Fe II (1,3-pdta)] (1,3-pdta = 1,2-N,Ν′- 1,3-propylenediaminetetraacetate) with molecular oxygen were studied as a function of [Fe II ], [O 2 ], pH, temperature and pressure. It was observed that the reaction with [Fe II (1,2-pdta)] is ca. 200 times faster than with the isomeric [Fe II (1,3-pdta)] complex under comparable conditions. In the presence of an excess of [Fe II (1,2-pdta)] three reaction steps could be identified for which the rate constants are = 3385 ± 607M -1 s -1 , k 2 = 226±9M -1 s -1 and k 3 = 111 ± 4M -1 s -1 at pH = 6 and T = 25°C. Since [Fe II (1,3-pdta)] is unstable during the reaction, only approximate rate constants for two of the electron-transfer steps could be determined, which are two orders of magnitude smaller than in the case of the [Fe II (1,2-pdta)] complex. Rate and activation parameters for the 1,2-pdta system are reported and discussed in reference to our earlier work on related systems.

For the following equations relating to the aquation and anation processes of cisplatin and similar squareplanar platinum(II) diamine complexes: rate constant (k -2 , Μ -1 s -1 ,25°C), activation enthalpy (ΔΗ # , kJ mol -1 ) , activation entropy (ΔS # , JΚ -1 mol -1 ) and activation volume (ΔV # , cm 3 mol -1 ) data have been found for the bromide anation of cis- [Pt(N) 2 (OH 2 ) 2 ] 2+ where (N) 2 = tn, en, cis-(NH 3 ) 2 , trans-(NH 3 ) 2 , Me 2 tn, chxn; CJS-(NH 3 ) 2 : k -2 = 0.45, ΔH # = 68, ΔS # = -25, trans-(NH 3 ) 2 : 4.23,56, -44, chxn: 1.13, 70, -9, Me 2 tn: 1.53, 65, -24, en: 1.29, 71, -5, ΔV # = -5.3, tn: 1.86,68, -12, ΔV # = -7.4. Further activation volumes (ΔV # , cm 3 mol -1 ) have been determined for selected k -1 and k -2 bromide or chloride anation data. k -1 (cm 3 mol -1 , CI - ): en -7.4. k -1 (cm 3 mol -1 , Br - ): cis-(NH 3 ) 2 -11, en -9.4, tn -7.66. k -2 (cm 3 mol -1 , CI - ): cis-(NH 3 ) 2 -5.1, en -5.7, tn -5.8. For all k x above where ΔV # has been measured, values lie in the range -4 to -11 cm 3 mol -1 and apart from two exceptions ΔS # is negative in 36 cases implying an associative mechanism for the substitution process at square-planar platinum(II) centres.

Transferrin is an iron carrier protein. It is known that the conformational change in the protein moiety induced by binding of nonsynergistic anions influences the metal release rate constants from metal transferrins. The copper dissociation rate constants of dicupric human(serum)-transferrin and ovo-transferrin were measured under the conditions of various pHs and ionic strengths to estimate the residues which interact with nonsynergistic anions. The pH and the ionic strength dependence of the copper dissociation rate constants in the N-lobe of serum (human)-transferrin and both lobes of ovo-Tf clearly show that some residue which has a pKa around 6.0 is involved in the binding of nonsynergistic anions and accelerating the copper dissociation rate. In the C-lobe of serum(human)-Tf, the copper dissociation rate constants were almost independent of pH between pH 7.0 and 9.0, but the dissociation rates were constantly accelerated by the presence of 0.1 Μ KCl. This behavior suggests that the residue in the C-lobe of serum(human)-Tf is constantly protonated between pH 7.0 and 9.0 and is involved in the binding of nonsynergistic anions binding site.

Under 254 nm irradiation cis-[Pt(C 6 H 5 CN) 2 Cl 2 ] is converted to H 2 PtCl 6 . Absorption of light by both the metal complex and the solvent contribute to the first step of this process, suggested to form HPt(C 6 H 5 CN) Cl 3 . A linear dependence of the reaction rate on light intensity appears to rule out chlorination by trichloromethyl radicals. However, at higher light intensities a higher order dependence on intensity develops, and under 313 nm irradiation is dominant, and a reaction between trichloromethyl radical and the excited state complex is proposed to account for this.

The kinetics and mechanisms of the reactions of aluminium(III) with ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA) have been investigated in aqueous solution at 25°C and ionic strength 0.5 mol dm -3 NaClO 4 . Al 3+ reacts with the H 4 DTPA - and H 3 DTPA2 - with rate constants of 2.06(±0.35) and 19.3(±1.6) respectively while Al(OH) 2+ reacts with H 3 DTPA 2- with a rate constant of 1.34(±0.20) × 10 3 dm 3 mol -1 s -1 . The outersphere association constant for reaction of Al 3+ with H 3 DTPA 2- was found to be 175(±38) mol -1 dm 3 . Al 3+ reacts with H 3 EDTA and H 2 EDTA 2- with rate constants of 4.73(±1.25) and 21.5(±2.21) dm 3 mol -1 s - 1 respectively. The outer-sphere association constant for reaction of Al 3+ with H 2 EDTA 2- is 164(±52.9) mol -1 dm 3 .

The kinetics of oxidation of tellurium(IV) (Te(IV)) by diperiodatonickelate(IV) (DPN) in aqueous alkaline medium has been studied spectrophotometrically. The reaction exhibits first order in [DPN] and fractional order both in [Te(IV)] and [OH-]. Periodate has a retarding effect on the rate of reaction. Effects of added products, ionic strength and dielectric constant of the reaction medium have been investigated. A mechanism involving the monoperiodatonickelate( IV) (MPN) as the reactive species of the oxidant has been proposed. The constants involved in the mechanism are obtained. There is a good agreement between the observed and calculated rate constants. The activation parameters are computed with respect to the slow step of the mechanism.

The complexation kinetics of fully deprotonated pendent arm macrocyclic ligands with Cu(II) hydroxo species had been investigated in aqueous solution at high pH. The pH dependence of the initial second order reaction is explained by the different reactivities for the tris- and tetra-hydroxo species of Cu(II). In some instances, the pendent arms accelerate the rates of the complex formation reaction. An intermediate complex is observed which reacts in a first order reaction to the final complex. The pH dependence of this reaction is the result of the existence of different numbers of aquo and hydroxo ligands additionally coordinated to the metal. Spectral changes for this second step are small, suggesting an isomerisation reaction is taking place.

The kinetics and mechanism of picolinic acid (PA)- catalysed oxidation of formic acid have been studied in aqueous HClO 4 media under the experimental conditions, 10 3 [Cr(VI)] T = 0.67-2.67 mol dm -3 , [PA] T = 0.01-0.12 mol dm -3 , [HCO 2 H] T = 0.1-0.8 mol dm -3 , [H + ] = 0.35-1.5 mol dm -3 ,1 = 1.5 mol dm -3 in the temperature range 25-40° C. The kinetic parameters of the catalysed path have been compared with those of the relatively slower uncatalysed path which proceed parallelly with the catalysed path. In the catalysed path, Cr(VI)-PA cyclic complex formed at the pre-equilibrium step receives a nucleophilic attack at the Cr(VI) centre by the substrate leading to a ternary complex with the release of a proton in a reversible path. Then the ternary complex experiences the redox decomposition at the rate-determining step giving rise to Cr(IV)-PA complex and CO 2 . Then the Cr(IV)-PA complex reacts faster ultimately giving rise to Cr(III)-PA complex. The anionic surfactant sodium dodecyl sulfate (SDS) catalyses both the uncatalysed and catalysed paths in aqueous HClO 4 media while the cationic surfactant cetyl pyridinium chloride (CPC) inhibits the reactions in aqueous H 2 SO 4 media. In the presence of the surfactants, applicability of the Piszkiewicz model has been tested for the ΡΑ-catalysed path. Micellar effects are in conformity with the proposed mechanism.

The kinetics and mechanism of chromium(III) catalysed oxidation of propan-1-ol and butan-1-ol by cerium(IV) in aqueous sulfuric acid media have been studied at different temperatures (30-40° C) under the conditions, [substrate] T (0.025-0.70 mol dm -3 ) × [Ce IV ]T (2.5-4.5 × 10 -3 mol dm -3 ) in the presence of [Cr] T = 0.0025-0.07 mol dm -3 . At fixed [H + ] and [HSO - 4 ] the observed rate law conforms to: -d ln[Ce IV ]/df =k obs =A[S] T [Cr] T /{B + C[Cr] T (D + E[S] T )}, where, A, B, C, D and Ε are constants expressed in terms of different rate constants and equilibrium constants; [S] T and [Cr] T denote the total substrate (i.e. propan-l-ol or butan-l-ol) and total chromium concentration respectively. The proposed mechanism involves the association of catalyst, substrate and oxidant at some pre-equilibrium steps before the electron transfer step. The overall process is found catalysed by H + . From the [HSO - 4 ] dependence, Ce(SO 4 ) 2 has been found kinetically active.