Nineteen new uraniumcomplexes, eighteen of which contain the element in the very unusual oxidation state of five, and one of which contains quadrivalent uraniumcomplexed for the first time by an arsenic donor ligand, are reported. The noveluranium(V) compounds include, for the first time, ligands whose donor atoms are nitrogen, phosphorus, arsenic, bismuth, sulfur, selenium and tellurium. The compounds