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 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.