Ligand effects outer-sphere reactions, kinetics

The catecholamines adrenaline, L-dopa, and dopamine, have the same oxidation stoicheiometry and show identical kinetic behaviour to catechol. Again no evidence was found for complex formation and the kinetic behaviour might be explained either by outer-sphere reactions of Mn + and MnOH + or by reactions of a weak complex which can exist in either of two forms differing by a proton. The ligand structure does, however, have little effect on the kinetic behaviour. 

Cobalt(II,III) sepulchrates have been used in the chemical education [415] and considerable number of the chemical and physicochemical studies as efficient quencher of the phosphorescence [416] and electronic excited states [417, 418], as a reductant in kinetic studies of redox reactions [419, 420], as a model for study of magnetodynamic [421], solvent [422] and pressure [423] effects on the outer-sphere electron-transfer reactions. Transfer chemical potentials (from solubility measurements) [424], electrochemical reduction potentials [425] and ligand-field parameters [426] for cobalt sepulchrates have been calculated. Solvent effect on Co chemical shift of cobalt(III) ion encapsulated in the sepulchrate cavity [427]... 

The biomethylation reaction between platinum and methylcobalamin involves both platinum(II) and platinum(IV) oxidation states. An outer-sphere complex is formed between the charged platinum(II) salts and the corrin macrocycle, which catalytically labilizes the Co—C o bond to electrophilic attack. A two-electron redox switch mechanism has been proposed between platinum(II) and platinum(IV). However, a mechanism consistent with the kinetic data is direct electrophilic attack by PtClg on the Co—C a bond in MeBu. Studies on [Pt(NH3)2(OH2)2] indicate that the bases on cobalt interact in the coordination sphere of platinum(II). Since both platinum(ll) and platinum(rV) are together required to effect methyl transfer from methylcobalamin to platinuni, Pt and C NMR spectroscopy have been used to show that the methyl group is transferred to the platinum of the platinum(n) reactant. The kinetics of demethylation by mixtures of platinum(II) and platinum(IV) complexes show a lack of dependence on the axial ligand. The authors conclude therefore that it is unlikely that the reaction involves direct attack by the bound platinum on the Co—C bond, and instead favor electron transfer from an orbital on the corrin ring to the boimd platinum group in the slow step, followed by rapid methyl transfer. ... 

Therefore, the effect of some ligands on the kinetics of reactions which proceed via the two mechanisms was studied reaction (50) was chosen as an example of a reaction which proceeds via the outer-sphere mechanism 105) and reactions (51)-(53) were chosen as examples of reactions which proceed via the inner-sphere mechanism 25,29,41,61). 

The inner-sphere reductions of [Co(NH3)5(SCONHR)] and [Co(NH3)5 (OCSNHR)] by Gr involve attack at the remote oxygen and sulfur atoms, respectively, with a subsequent isomerization of the 0-bonded ehromium(III) product in the former reaction. The unusually rapid reactions of the S-bonded cobalt(III) complexes are attributed to a structural tran -effect on the Co—N bond length, reducing the reorganization energy needed to form the transition state. A kinetic study of the Cr reduction of [Co(NH3)5(pyruvate)] reveals that the rate of reduction is dependent on the nature of pyruvate ligand, with the keto form about 400 times as reactive as the hydrated form. An inner-sphere mechanism has be postulated for the Cr reduction of [Co(NH3)5(pyridine N-oxide)] on the basis of the rate and activation parameters. The outer-sphere Cr reduction of [Co(sepulchrate)] is catalyzed by halide ions, with the ion-pair formation constants for [Co(sep), estimated to be 5.5, 2.3, and 1.7 M" for Cl", Br", and I", respectively. ... 

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