Effect of Nucleophiles on the Rates of Dissociation and Racemization of the Tris(1,10-phenanthroline)iron(II) Ion in Aqueous Solutions

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Abstract The rates of dissociation and racemization of the tris(1,10-phenanthroline)iron(II) ion were obtained in aqueous solutions containing fourteen kinds of nucleophile. They ranged from 0.7 to 9.3 and from 0.9 to 3.2 times the rates in pure water, respectively. The results were analyzed by considering simultaneous reactions of free and ion-paired complex ions. The rate constants of the dissociation (ligand release) of the complex existing in ion-pairs increased in the following order of the counter-ions: I−<Br−<Cl−<(H2O)<NO2−<CH3COO−<OCN−<N3−<F−<OH−≈CN−; here H2O is regarded as a nucleophile for the free complex ion. This order is similar to that of proton affinities of nucleophiles. The results show that the dissociation is accelerated by σ donation from the nucleophile, suggesting an ion-pair interchange mechanism. The intramolecular racemization rate constant, given by total recemization rate constant minus dissociation rate constant, increased in the following order of nucleophiles: OH−≈CN−≈F−≈Cl−<Br−≈NO2−<(H2O)<CH3COO−<OCN−<I−<N3−<(thiourea)≈SCN−≈SeCN−<(pyridine). This suggests that the polarizability of nucleophiles has a greater effect than their basicity. The operation of π interaction between the complex ion and the nucleophile in the transition state is also suggested. The ion-association constants obtained from the rate data were in the range of 1 to 8 (I=1.0, 32.0 °C).