COMPLEX-FORMATION KINETICS OF IRON(III) WITH CHLORITE ION IN AQUEOUS-SOLUTION - MECHANISTIC INFORMATION FROM PRESSURE EFFECTS

The complex formation kinetics between iron(III) and chlorite ion was investigated as a function of pressure at 5-degrees-C and 1.0 M NaClO4 by using a stopped-flow technique. As auxiliary data, the stability constant and molar reaction volumes were determined for the equilibria H+ + ClO2- half arrow right over half arrow left HClO2 (log K(p) = 1.56 +/- 0.04, DELTAV(p)BAR = +10.7 +/- 1.5 cm3 mol-1) and Fe3+ + ClO2- half arrow right over half arrow left FeClO22+ (log K1 = 1.12 +/- 0.05, DELTAV1BAR = +16.5 +/- 2.7 CM3 Mol-1). It was shown that the complex formation predominantly occurs via the conjugate acid/conjugate base pathway according to an I(d) mechanism: Fe(OH)2+ + Hclo2 --> FeClO22+ + H2O, where k1' = (3.0 +/- 0.2) x 10(3) M-1 s-1 and DELTAV1(double dagger) = +6.9 +/- 2.3 CM3 Mol-1. A comparison of the results with literature data revealed an analogy between the formation mechanisms of complexes FeClO22+ and FeN32+. In contrast, the mechanism of the formation of the FeNCS2+ complex was described as I(a) before. This mechanistic changeover is interpreted in terms of the nucleophilic strength of the ligands. It is suggested that, in general, the mechanism of complex-formation reactions of iron(III) can be controlled by the nucleophilicity of the donor group on the entering ligand. Mechanistic implications of the results for the iron(III)-catalyzed decomposition of chlorite ion are also discussed.