Identification of the +2 oxidation state for uranium in a crystalline molecular complex, [K(2.2.2-Cryptand)][(C₅H₄SiMe ₃)₃U]

Flash reduction of Cp′₃U (Cp′ = C₅H ₄SiMe₃) in a column of potassium graphite in the presence of 2.2.2-cryptand generates crystalline [K(2.2.2-cryptand)] [Cp′₃U], the first isolable molecular U²⁺ complex. To ensure that this was not the U³⁺ hydride, [K(2.2.2-cryptand)] [Cp′₃UH], which could be crystallographically similar, the hydride complex was synthesized by addition of KH to Cp′₃U and by reduction of H₂ by the U²⁺ complex and was confirmed to be a different compound. Density functional theory calculations indicate a 5f³6d¹ quintet ground state for the [Cp′ ₃U]^- anion and match the observed strong transitions in its optical spectrum.