KINETICS OF REACTIONS OF AQUEOUS IRON(III) IONS WITH AZIDE AND THIOCYANATE AT HIGH-PRESSURES

A high-pressure stopped-flow study of the reaction of Fe(III)(aq) with HN3/N3- at 25-degrees-C and ionic strength 1.0 mol kg-1 showed that two pathways predominated: (a) the previously unreported pathway involving Fe(H2O)5OH2+ and N3- (rate constant k(bb) = 3.8 X 10(5) kg mol-1 s-1, volume of activation DELTAV(bb)double dagger = +12.9 +/- 1.5 cm3 mol-1) and (b) the reaction of Fe(H2O)5OH2+ with HN3(k(ba) = 5.0 X 10(3) kg mol-1 s-1, DELTAV(ba)double dagger = +6.8 +/- 0.5 cm3 mol-1). A third pathway involving Fe(H2O)63+ and HN3 was barely detectable. The isomeric alternative to pathway b, in which Fe(H2O)63+ would react directly with N3-, can be ruled out by consideration of these kinetic data relative to those for reactions of Fe(III)(aq) with other nucleophiles, notably NCS-. In this context, the latter reaction has been reexamined by stopped-flow spectrophotometry at pressures up to 200 MPa to resolve serious discrepancies existing in the literature; for Fe(H2O)63+ + NCS-, DELTAV(ab)'double dagger = -5.7 +/- 0.3 cm3 mol-1 and the equilibrium molar volume change is +11.8 +/- 0.3 cm3 mol-1, while for Fe(H2O)5OH2+ +NCS-, DELTAV(bb)'double dagger = +9.0 +/- 0.4 cm3 mol-1, agreeing with the stopped-flow results of Funahashi et al. (Inorg. Chem. 1983, 22, 2070) but contradicting the results obtained by several groups using temperature- or pressure-jump methods. Possible sources of these discrepancies are considered. The mechanisms of these reactions of Fe(H2O)5OH2+ may be described as I(d), and those of Fe(H2O)63+ as I(a), within the limitations of this classification.