Why Is Uranyl Formohydroxamate Red?

The complexation of UO22+ by formohydroxamate (FHA–) creates solutions with dark red coloration. The inherent redox activity of formohydroxamate leads to the possibility that these solutions contain U­(V) complexes, which are often red. We demonstrate that the reaction of U­(VI) with formohydroxamate does not result in reduction, but rather in formation of the putative cis-aquo UO2(FHA)2(H2O)2, whose polymeric solid-state structure, UO2(FHA)2, contains an unusually bent UO22+ unit and a highly distorted coordination environment around a U­(VI) cation in general. The bending of the uranyl cation results from unusually strong π donation from the FHA– ligands into the 6d and 5f orbitals of the U­(VI) cation. The alteration of the bonding in the uranyl unit drastically changes its electronic and vibrational features.