ACTINIDE COMPLEXATION KINETICS - RATE AND MECHANISM OF DIOXONEPTUNIUM(V) REACTION WITH CHLOROPHOSPHONAZO-III

The rates of complex formation and dissociation in the system NpO2+-Ch lorophosphonazo III enzeneazo)-1,8-dihydroxynaphthalene-3,6-disulfonic acid-CLIII) has been investigated by stopped flow spectrophotometry. In addition, limited studies have been made of the rates of reaction o f La3+, Eu3+, Dy3+, and Fe3+ with CLIII. The rate determining step in each system is an intramolecular process, the NpO2+-CLIII reaction pro ceeding by a first order approach to equilibrium in the acid range fro m 0.1 to 1.0 M. Complex formation occurs in a process independent of a cidity, while both acid dependent and independent dissociation pathway s are observed. The activation parameters for the complex formation re action are Delta H = 46.2(+/-0.3) kT mol(-1) and Delta S* = 7(+/-1) J mol(-1) K-1 (I = 1.0 M). The corresponding parameters for the acid de pendent and independent dissociation pathways are Delta H = 38.8(+/-0 .6) kJ mol(-1), Delta S = -96(+/-18) J mol(-1) K-1, Delta H*: = 70.0( +/-0.1) kJ mol(-1) and Delta S = 17(+/-1) J mol(-1) K-1, respectively . An isokinetic relationship is observed between the activation parame ters for CLIII complex formation with NpO2+, UO22+, Th4+, and Zr4+. Th e rates of CLIII complex formation reactions for Fe3+, Zr4+, NpO2+, UO 22+, Th4+, La3+, Eu3+, and Dy3+ correlate with the cation radius rathe r than the charge/radius ratio.