Surprises in the Solvent-Induced Self-Ionization in the Uranium Tetrahalide UX(4) (X = Cl, Br, I)/Ethyl Acetate System

[Image: see text] The reaction of the uranium(IV) halides UCl(4), UBr(4), or UI(4) with ethyl acetate (EtOAc) leads to the formation of the complexes [UX(3)(EtOAc)(4)][UX(5)(EtOAc)] (X = Cl, Br) or [UI(4)(EtOAc)(3)]. Thus, both UCl(4) and UBr(4) show self-ionization in ethyl acetate to a distorted pentagonal bipyramidal [UX(3)(EtOAc)(4)](+) cation and a distorted octahedral [UX(5)(EtOAc)](−) anion. Surprisingly, the chloride and bromide compounds are not isotypic. While [UCl(3)(EtOAc)(4)][UCl(5)(EtOAc)] crystallizes in the orthorhombic crystal system, space group P2(1)2(1)2(1) at 250 K, the bromide compound crystallizes in the monoclinic crystal system, P12(1)/n1 at 100 K. Unexpectedly, UI(4) does not show self-ionization but forms [UI(4)(EtOAc)(3)] molecules, which crystallize in the monoclinic crystal system, P2(1)/c, at 100 K. The compounds were characterized by single-crystal X-ray diffraction, IR, Raman, and NMR spectroscopy, as well as molecular quantum chemical calculations using solvent models.