Mechanisms of ligand substitution reactions in ternary dioxouranium(VI) complexes

The activation parameters of various inter- and intramolecular ligand exchange reactions in ternary complexes of the type UO2LF3 (where L is one of the bidentate ligands picolinate, 4-nitropicolinate, 4-(3-pentyl)picolinate, oxalate, or carbonate) and UO2L2F (where L = picolinate or oxalate) have been determined by different NMR techniques. The activation entropies for the reactions have been used to discuss their intimate mechanism, particularly the solvent participation. Additional mechanistic information has been obtained from studies of H+/D+ catalysis of the various reactions. Through the use of the proton-catalyzed pathway, the rate of carbonate exchange in UO2CO3F33- was varied by a factor of ca. 10. The fact that the rate of fluoride exchange remained constant clearly indicates that the exchanges of carbonate and fluoride follow parallel pathways. The activation entropies of most of the fluoride exchange reactions in UO2LF3 have values close to 10 J/K mol, indicating dissociative (D) or dissociative interchange (ID) mechanisms. The exchange of L takes place in two steps: The first is a chelate ring opening/closing, resulting in a ligand rotation. The second is bond breaking at the carboxylate end and exchange; hence, the activation entropies for the exchange vary with L