EFFECT OF PRESSURE ON THE COMPLEX-FORMATION AND AQUATION KINETICS OF IRON(III) WITH HYDROXAMIC ACIDS

The activation and reaction volumes for the formation and aquation of (acethydroxamato)iron(III) complexes, as well as the activation volumes for the formation of the (desferrioxamine B)iron(III) complex, have been obtained by high-pressure stopped-flow and UV-vis spectral measurements. The data indicate a gradual mechanistic changeover from I(a) to I(d) for the stepwise proton-catalyzed hydrolysis of the tris(acethydroxamato)iron(III) complex and vice versa for the corresponding formation reactions. The activation volumes for the complexation of Fe(H2O)6(3+) and Fe(H2O)5(OH)2+ with both acethydroxamic acid (HA) and desferrioxamine B in its fully protonated form (H4dfb+) exhibit opposite signs, indicating associative and dissociative modes of activation, respectively. The obtained results suggest that the substitution behavior of the Fe(III) complexes is controlled by the presence of OH- or A- in the coordination sphere.