Electrostatic Effect of Polymer-Co HI Complexes

Chelating agents edta ethylenediaminetetraacetic acid, edtaoh = hydroxyethylethylenediaminetriaceticacid, cydta = cyclohexanediaminetetraacetic acid, hdtpa = protonated diethylenetriaminepentaacetic acid, [Co(III)(en)2(PVP)Cl]Cl x ethyl 0.45, DP of PVP 98. 

This reaction profile of the polymer complex has some similarities with the phenomenon of the polyelectrolyte-catalyzed reactions. It has been reported that the reactions between two positively charged species in aqueous solution are drastically accelerated in the presence of polyanionsS2 84 For example, the electron-transfer reaction between [Co(IIIXen)2(Py)Cl]2+ and [Fe(IIXOH2)6]2+ is very slow because the reaction occurs between two cations however, the addition of a small amount of poly(styrenesulfonate) accelerates the reaction by a factor of 103 84). This result is also interpreted as indicating that the two positively charged reactants are both concentrated in the polyanion domain, so that they encounter each other more frequently [Fig. 17(b)]. 

The activation parameters are presented in Table 819 For the reactions be between the Co(III) complex2+ and Fe-edta2-, (a) to (c) in Table 8, the activation enthalpy is smaller and the activation entropy larger than for the reduction by Fe2+, (d) to (f), which is a reaction of two cations. A comparison of the parameters for the polymer complex, (b) or (c), with those for the pyridine complex, (a) shows that the acceleration for the PVP or QPVP complex is based on a decrease in activation enthalpy and an increase in activation entropy. This is the opposite of the polyelectrolyte-catalyzed reaction, in which the acceleration is due to an increase in activation entropy (compare(e) with (d)). In the polyelectrolyte-catalyzed system the acceleration and increase in activation entropy are attributed to the increase in the local concentration of the two reactants, the Co(HI)-Py complex2 and Fe2+ 84, whereas in the reaction of the polymer complex the large activation entropy and small activation enthalpy are held to be due to the increase in the local concentration of the reactant Fe(II)-edta2 and the electrostatic attraction between the reactant and the Co(III) complex, which is fixed to the polycation chain. 

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