KINETICS AND MECHANISMS OF THE COMPLEXATION OF AQUEOUS LANTHANIDE IONS BY 4-(2-PYRIDYLAZO)RESORCINOL

Three kinetic steps were observed for the complexations of Eu3+ and UO 22+ by PAR [4-(2-pyridylazo)resorcinol] or PAN [1-(2-pyridylazo)-2-nap hthol] in different buffered solutions. The first step can be assigned to the co-ordination of the nitrogen donor from the pyridine moiety o f the ligand based on the dependencies of [Eu3+], pH, pressure, nature and concentration of the buffer. The rate-determining step is the rel ease of water molecules from the co-ordination sphere of the lanthanid e ion. Variations in the rate of the first step with different lanthan ide ions indicated that a co-ordination number changeover is involved in this lanthanide series. For the second step the formation of a ''hy droazone-Ln(3+) chelate'' intermediate accounts for all of the observe d kinetic behaviors. The kinetic investigations of the third step show that there is a deprotonation preequilibrium preceding the transition state of the final product with two chelated 5-membered rings involve d. Surprisingly, the rate constants of the three steps for the complex ation of UO22+ by PAR are very close to those for 18-crown-6 and diaza -18-crown-6 reacting with uranyl ion. The differences in the kinetics between PAR and PAN can be related to the difference in their structur es.