Isolation of a Perfectly Linear Uranium(II) Metallocene.

Reduction of the uranium(III) metallocene [(eta(5)-(C5Pr5)-Pr-i)(2)UI] (1) with potassium graphite produces the "second-generation" uranocene [(eta(5)-(C5Pr5)-Pr-i)(2)U] (2), which contains uranium in the formal divalent oxidation state. The geometry of 2 is that of a perfectly linear bis(cyclopentadienyl) sandwich complex, with the ground-state valence electron configuration of uranium(II) revealed by electronic spectroscopy and density functional theory to be 5f(3) 6d(1). Appreciable covalent contributions to the metal-ligand bonds were determined from a computational study of 2, including participation from the uranium 5f and 6d orbitals. Whereas three unpaired electrons in 2 occupy orbitals with essentially pure 5f character, the fourth electron resides in an orbital defined by strong 7s-6dz2 mixing.