The rate and mechanism of complexation reactions of neptunyl(V) and uranyl(
VI) by oxydiacetic acid at pH 3 in 1.0 M NaClO4 solutions has been investig
ated by stopped-flow spectrophotometry. This tridentate ligand is able to a
ccommodate the structural requirements of the linear dioxoactinide cations,
and is known to form moderately stable complexes with these metal ions. As
was reported previously in a study of diphosphonate complexes of these met
al ions, the rate of formation of the respective 1:1 complexes is faster fo
r Np(V) than U(VI), despite a stronger thermodynamic driving force for the
latter. This result further supports the proposition that the rate of solve
nt and ligand rearrangement in the precursor complex is an important parame
ter in determining the rate of actinyl complexation reactions. The applicab
ility of Marcus reaction rate theory, as modified by Albery, to complexatio
n reactions of this type is discussed.