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Title: Kinetics and mechanism of the reaction of trans-(diaqua)(N,N' - ethylene bis-(salicylidineiminato)cobalt(III) with ascorbic acid
Authors: Dash, Anadi C
Das, Arabinda
Bramha, Gouri S
Mohanty, Prakash
Issue Date: Nov-1998
Publisher: NISCAIR-CSIR, India
Abstract: The kinetics of the reactions of trans-[Co(Salen)(OH2)2]+ (Salen = N.N¢-ethylene bis (salicylidineiminate) ·jth ascorbic acid (H2Asc) have been studied under varying conditions of pH, [ascorbic acid]T, and temperature at .5 mol dm-3 ionic strength. The initial fast reactions observed in the stopped flow time scale are due to the complex formation between the reactants. This occurs in two phases i.e., the formation of the trans- [(2 ua)(ascorbato)CoIII(Salen)] and its transformation to the corresponding ascorbate chelate. The rate constants are the activation parameters for the formation of the monobonded and chelate ascorbate complexes are reported. T low values of ΔH+ and negative values of ΔS+ for the complexation reaction favour associative interchange mechanism(la). The hydroxide in trans-[Co(Salen)(OH)(OH2)] marginally accelerates substitution of the aqua and by HAsc- and the trans-[Co(Salen)(OH)(AscH)]- is considered to undergo fast internal proton transfer to .nerate trans-[Co(Salen)(OH2)(Asc)]- which undergoes chelation of the Colll centre by the bound ascorbate noiety; the latter reaction is, however, 15 times slower than the corresponding reaction of trans- [Co(Salen)(OH2)(AscH)]. The faster complexation reactions are followed by the slow redox reactions. The rate constant for the internal reduction of Colll by the coordinated ascorbate in the chelate [Co(Salen)(AscH)] is 5 - 10 times (25°C45°C) faster than the same for [Co(Salen)(Asc)]-. This trend in reactivity is due to the low value of ΔH for the former although the high negative value of ΔS compensates at least partly the overriding effect of the activation enthalpy. The internal redox occurs via innersphere mechanism. We also have observed a redox path involving trans-[Co(Salen)(OH2)(AscH2)]+ and H2Asc for which electron transfer most likely involves outersphere mechanism.
Page(s): 947-960
ISSN: 0975-0975(Online); 0376-4710(Print)
Appears in Collections:IJC-A Vol.37A(11) [November 1998]

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