Volume 1, Issue 4

Hypochlorite oxidation of the Schiff base complex of chromium(IIl), Cr(salen)(H 2 0) 2 has been investigated at pH 9.0 to understand the mechanism of formation of oxochromium(V) species, which has been implicated in the apoptotic cell death brought about by Cr(salen)(H 2) O 2 . The reaction exhibited a two stage kinetic profile due to the formation of oxochromium( V) species and its subsequent decay. The final product has been unambiguously identified as Chromate. The overall stoichiometry of lfCr(III)]: 1.5[OCl ] has been established for the reaction. The thermodynamic parameters for the formation of oxochromium( V) species are ΔΗ 9.9 ± 1.8 Kcal mol 1 and ΔS 24.5 ± 8.0 e.u. Reaction schemes have been proposed invoking Cr(V) and Cr(IV) dimeric species as possible intermediates. An inner sphere mechanism has been invoked in the formation of oxochromium( V) intermediate.

A formula for the calculation of formation constants for ion-triplets in contact, KIT, is derived on the basis of a thermodynamic argument. This approach also yields an expression for the association constants for ion-pairs, ΚIP, within the same approximation, leading to a modification of the familiar Fuoss equation. The formula for K1T is compared with literature data for transition metal cyano complexes.

The kinetics and mechanism of the oxidation of hydroxylamine by tris(pyridine-2-carboxylato)manganese( III) in Na(pic)-picH [where Na( pic) = sodium salt of pyridine-2-carboxylic acid (picH)] has been studied in the pH range 4.89-6.10. The rate of the reaction increases with increasing [substrate] but with decreasing acidity. A rate expression has been derived from which the dissociation constant of the equilibrium NH 3 OH + ⇄ NH 2 OH + H + has been computed at different temperatures. Activation parameters for the slow rate determining step have been evaluated. Manganese(III) complex is reduced by the hydroxylammonium ion by an inner sphere mechanism. The reaction appears to proceed through the intermediate formation of free radicals to give products of oxidation.

The water-soluble complex RUCl 2 (TPPTS) 3 has been prepared. In aqueous solution the complex undergoes both CI and TPPTS dissociation, but has been characterized in solution. The reaction with H 2 occurs readily through at least two separate mechanisms. The slower step is independent of [CI ] and probably involves direct reaction of H 2 with RUCl 2 (TPPTS) 3 . A much more rapid step is strongly inhibited by [CI"] and probably involves CI dissociation, perhaps two CI dissociations. In the presence of excess CI the kinetics are well-behaved showing a first-order dependence on [RUCl 2 (TPPTS) 3 ] and [H 2 ].

The reactions of [Pt(dipic)Cl] (where H 2 dipic = pyridine-2,6-dicarboxylic acid) with 1-methylimidazole, ethylenediamine and nicotinic acid were studied at 40°C in dimethylformamide medium spectrophotometrically. The successive entry of two 1- methylimidazole molecules through initial chloride ion replacement followed by carboxylate ring opening process was further verified through intermediate product isolation. Nicotinic acid shows only the chloride substitution reaction. Bidentate ethylenediamine ligand gets chelated with the platinum(II) complex through slow chloride replacement and rapid ring opening process. All the products have been isolated and characterised through 1 H NMR spectra.

The title reaction proceeds through the formation of an intermediate [Co III W... N 3 ] 6 formed between [Com III W] 5- and N 3 - and not between [Co III W] 5 and HN 3 . The absorbance measurements of the intermediate at various [HN3] yield an equilibrium constant Κ = 0.012 ± 0.001 at 35° C which compares well with the value 0.011 obtained at that temperature from the kinetics data. The reaction is first-order with respect to [Com III W] 5- and HN 3 . The pH dependence of k obs , the pseudo-first-order rate constant ([HN 3 ] > [Co IIl W] 5- ), is consistent with the linearity of the plots between k obs -1 and [H + ] with intercepts on the rate ordinate. The inverse correlation between k obs and [H + ] is traced to the equilibrium HN 3 ⇄ N - 3 + H + followed by [Co III W] 5 + N 3 ⇄ [Co III W... N 3 ]6 andnot through the equilibria [Co IIl W] 5 + HN 3 ⇄ [Co lII W ...N 3 ] 6- + H + . The high reactivity of the N - 3 ion is traced to the presence of an unshared pair of electrons as in the case of NH 2 OH and N 2 H 4 which seems to imply an inner-sphere mechanism with the substrate binding to the oxidant via this pair of electrons. However, in view of the well-protected nature of the central Co III atom in [Co lII W] 5- ion there is difficulty in visualising the coordination of the substrate with the Co1" ion. That the electron transfer is outer-sphere is substantiated by excellent agreement between the experimental rate of electron-transfer, 0.305 dm 3 mol -1 s -1 , and the one (0.339 dm 3 mol -1 s -1 ) calculated by the application of Marcus equations.

The oxidation of thiocyanate by periodate has been studied in alkaline media. A microamount of Os( VIII) is sufficient to catalyse the reaction. The active species of catalyst and oxidant in the reaction are understood to be OsO 4 (OH) 2 2- and IO - 4 . The autocatalysis exhibited by one of the products i.e., cyanate is attributed to adduct formation between cyanate and periodate. A composite mechanism and rate law are proposed. The reaction constants involved in the mechanism are evaluated.

The kinetics of the reaction of [Cu(bigH) 2 ] 2+ (bigH = biguanide) with an excess of different amino acids (LH) like ß-alanine, L-phenylalanine and L-valine in aqueous solution in the 7.60-9.00 pH range at different temperatures (30-40°C) have been studied by stopped-flow spectrophotometry and compared with those of different ligands reacting with the title copper(II) complex. The ligand replacement process has been suggested to pass through an intermediate formation of the ternary complex, [Cu(bigH)L] + as the rate determining step, followed by the rapid transformation into the binary complex, [CuL 2 ]. The overall ligand replacement process follows a two-term rate law, k obs = k 0 + k 1 K a [L] T /([H + ] + K a , where [L] T gives the total concentration of the entering ligand, i.e. amino acid, and Κ a gives the deprotonation constant of the amino acid (LH). Thus the process experiences both the ligand independent path (i.e. k0) and ligand dependent path (i.e. k 0 ). The solvent assisted k0 path leads to a copper(II) mono-biguanide complex followed by the rapid nucleophilic substitution; the k1 path is in agreement with an associative mechanism. The k 0 path is independent of the structure and basicity of the entering amino acids while the k 0 path is dependent on the basicity and steric crowding of the entering amino acid. The reactivity order is: glycine > α-alanine > ß-alanine > L-valine > L-phenylalanine. The activation parameters have been evaluated. An iso-kinetic trend for different amino acids and other nucleophiles reacting with the title copper(II) complex has been found to prevail.

The kinetics of aquation and base hydrolysis reactions of fmns-[Co(NH 3 ) 4 (NH 2 CH 3 )(OSO 3 )] + have been studied. In acid solution the aquation rate, R aq , follows the equation R aq /[complex] = k s + k c [H + ], at constant ionic strenght μ = 1.0Μ. The activation parameters are ΔΗ* = 88.8 kj mol -1 , ΔS* = - 50.5 JK -1 mol -1 , ΔΗ* = 96.7 kj mol -1 and ΔS* =-23 JK -1 mol -1 . The rate constants at 25°C are k s = 4.15 χ 10 -6 s -1 and k c = 4.52M -1 s -1 . The rate of base hydrolysis, R OH , follows the equation R OH/ complex] = k 0H [OH - ]. The activation parameters are ΔΗ* OH =74.9kJmol -1 and ΔS* OH = 2JK -1 mol -1 and the rate constant is k OH = 0.58M 1 s -1 at 25°C and μ, = 0.15 Μ. The stereochemistry of the hidroxo product has been determined (cis-[Co(NH 3 ) 4 (NH 2 CH 3 )- (OH)] 2+ =9%). The results are discussed in the light of the reaction mechanisms proposed so far.