Covalency-Driven Dimerization of Plutonium(IV) in a Hydroxamate Complex

The reaction of formohydroxamic acid [NH­(OH)­CHO, FHA] with PuIII should result in stabilization of the trivalent oxidation state. However, slow oxidation to PuIV occurs, which leads to formation of the dimeric plutonium­(IV) formohydroxamate complex Pu2(FHA)8. In addition to being reductants, hydroxamates are also strong π-donor ligands. Here we show that formation of the Pu2(FHA)8 dimer occurs via covalency between the 5f orbitals on plutonium and the π* orbitals of FHA– anions, which gives rise to a broad and intense ligand-to-metal charge-transfer feature. Time-dependent density functional theory calculations corroborate this assignment.