An Inverted-Sandwich Diuranium μ-η(5):η(5)-Cyclo-P(5) Complex Supported by U-P(5) δ-Bonding**

Reaction of [U(Tren(TIPS))] [1, Tren(TIPS)=N(CH(2)CH(2)NSiiPr(3))(3)] with 0.25 equivalents of P(4) reproducibly affords the unprecedented actinide inverted sandwich cyclo-P(5) complex [{U(Tren(TIPS))}(2)(μ-η(5):η(5)-cyclo-P(5))] (2). All prior examples of cyclo-P(5) are stabilized by d-block metals, so 2 shows that cyclo-P(5) does not require d-block ions to be prepared. Although cyclo-P(5) is isolobal to cyclopentadienyl, which usually bonds to metals via σ- and π-interactions with minimal δ-bonding, theoretical calculations suggest the principal bonding in the U(P(5))U unit is polarized δ-bonding. Surprisingly, the characterization data are overall consistent with charge transfer from uranium to the cyclo-P(5) unit to give a cyclo-P(5) charge state that approximates to a dianionic formulation. This is ascribed to the larger size and superior acceptor character of cyclo-P(5) compared to cyclopentadienyl, the strongly reducing nature of uranium(III), and the availability of uranium δ-symmetry 5f orbitals.