An Inverted-Sandwich Diuranium μ-η⁵:η⁵-Cyclo-P₅ Complex Supported by U-P₅ δ-Bonding

Reaction of [U(Tren^TIPS)] [1, Tren^TIPS=N(CH₂CH₂NSiiPr₃)₃] with 0.25 equivalents of P₄ reproducibly affords the unprecedented actinide inverted sandwich cyclo-P₅ complex [{U(Tren^TIPS)}₂(μ-η⁵:η⁵-cyclo-P₅)] (2). All prior examples of cyclo-P₅ are stabilized by d-block metals, so 2 shows that cyclo-P₅ does not require d-block ions to be prepared. Although cyclo-P₅ 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(P₅)U unit is polarized δ-bonding. Surprisingly, the characterization data are overall consistent with charge transfer from uranium to the cyclo-P₅ unit to give a cyclo-P₅ charge state that approximates to a dianionic formulation. This is ascribed to the larger size and superior acceptor character of cyclo-P₅ compared to cyclopentadienyl, the strongly reducing nature of uranium(III), and the availability of uranium δ-symmetry 5f orbitals.