Why Is Uranyl Formohydroxamate Red?

Abstract

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 <i>cis</i>-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 6<i>d</i> and 5<i>f</i> orbitals of the U­(VI) cation. The alteration of the bonding in the uranyl unit drastically changes its electronic and vibrational features