Identification of the +2 oxidation state for uranium in a crystalline molecular complex, [K(2.2.2-cryptand)][(C5H4SiMe3)3U].

Flash reduction of Cp'3U (Cp' = C5H4SiMe3) 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 U(2+) complex. To ensure that this was not the U(3+) 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 U(2+) complex and was confirmed to be a different compound. Density functional theory calculations indicate a 5f(3)6d(1) quintet ground state for the [Cp'3U](-) anion and match the observed strong transitions in its optical spectrum.