Role of ligand basicity and-stereochemistry in the extraction of plutonium(IV) isoxazolonates

The extraction behaviour of plutonium (IV) was investigated from nitric acid and perchloric acid using 3-phenyl-4-benzoyl-5-isoxazolone (HPBI) as well as a mixture of HPBI and tri-n-octyl phosphine oxide (TOPO) in xylene. Studies on the binary extraction system Pu(IV)-HPBI indicated extraction of species of the type Pu(PBI)(4). In the presence of TOPO, while species of the type Pu(PBI)(3) (ClO4) (.) (TOPO) and Pu(PBI)(4)(TOPO)-T-. were co-extracted from perchloric acid medium, a mixture of Pu(PBI)(2)(NO3)(2)(.)2TOPO, Pu(PBI)(3)(NO3)(.)(TOPO) and Pu(PBI)(4)(TOPO)-T-. were co-extracted from nitric acid medium. In addition to the electronic factors, steric factors were found to play a significant role during the formation of binary/ternary/quaternary complexes. The steric hindrance due to the unusual conformation of the isoxazolone led to lowering of the equilibrium constants for both the binary and ternary extraction systems. Possible steric factor influences are explained with the help of molecular modelling.