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.