Identification of Oxidation State +1 in a Molecular Uranium Complex

[Image: see text] The concept of oxidation state plays a fundamentally important role in defining the chemistry of the elements. In the f block of the periodic table, well-known oxidation states in compounds of the lanthanides include 0, +2, +3 and +4, and oxidation states for the actinides range from +7 to +2. Oxidation state +1 is conspicuous by its absence from the f-block elements. Here we show that the uranium(II) metallocene [U(η(5)-C(5)(i)Pr(5))(2)] and the uranium(III) metallocene [IU(η(5)-C(5)(i)Pr(5))(2)] can be reduced by potassium graphite in the presence of 2.2.2-cryptand to the uranium(I) metallocene [U(η(5)-C(5)(i)Pr(5))(2)](−) (1) (C(5)(i)Pr(5) = pentaisopropylcyclopentadienyl) as the salt of [K(2.2.2-cryptand)](+). An X-ray crystallographic study revealed that 1 has a bent metallocene structure, and theoretical studies and magnetic measurements confirmed that the electronic ground state of uranium(I) adopts a 5f(3)(7s/6d(z(2)))(1)(6d(x(2)–y(2))/6d(xy))(1) configuration. The metal–ligand bonding in 1 consists of contributions from uranium 5f, 6d, and 7s orbitals, with the 6d orbitals engaging in weak but non-negligible covalent interactions. Identification of the oxidation state +1 for uranium expands the range of isolable oxidation states for the f-block elements and potentially signposts a synthetic route to this elusive species for other actinides and the lanthanides.