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(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)(mu-eta(5) : eta(5)-cyclo-P-5)] (2). All prior examples of cyclo-P-5 are stabilized by delta-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 sigma -and pi-interactions with minimal delta-bonding, theoretical calculations suggest the principal bonding in the U(P-5) U unit is polarized delta-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 delta-symmetry 5f orbitals.