A Revised Structure Model for the UCl6 Structure Type,Novel Modifications of UCl6 and UBr5, and a Comment on the Modifications of Protactinium Pentabromides

The redetermination of the crystal structure of trigonal UCl₆, which is the eponym for the UCl₆ structure type, showed that certain atomic coordinates had been incorrectly reported. This led to noticeably different U−Cl distances within the octahedral UCl₆ molecule (2.41 and 2.51 Å). Within the revised structure model presented here, which is based on single crystal data as well as quantum chemical calculations, all U−Cl distances are essentially equal within standard uncertainty (2.431(5), 2.437(5), and 2.439(6) Å). This room temperature modification, called rt-UCl₆, crystallizes in the trigonal space group P m1, No. 164, hP21, with a=10.907(2), c=5.9883(12) Å, V=616.9(2) ų, Z=3 at T=253 K. A new low-temperature (lt) modification of UCl₆ is also presented that was obtained by cooling a single crystal of rt-UCl_6. The phase change occurs between 150 and 175 K. lt-UCl₆ crystallizes isotypic to a low-temperature modification of SF₆ in the monoclinic crystal system, space group C2/m, No. 12, mS42, with a=17.847(4), b=10.8347(18), c=6.2670(17) Å, β=96.68(2)°, V=1203.6(5) ų, Z=6 at 100 K. The Cl anions form a close-packed structure corresponding to the α-Sm type with uranium atoms in the octahedral voids. During the synthesis of UBr₅ a new modification was obtained that crystallizes in the triclinic crystal system, space group P , No. 2, aP36, with a=10.4021(6), b=11.1620(6), c=12.2942(7) Å, α=68.3340(10)°, β=69.6410(10)° and γ=89.5290(10)°, V=1231.84(12) ų, Z=3 at T=100 K. In this structure the UBr₅ units are dimerized to U₂Br₁₀ molecules. The Br anions also form a close-packed structure of the α-Sm type with adjacent uranium atoms in the octahedral voids. Comparisons of the crystal structures of the compounds MX₅ (M=Pa, U; X=Cl, Br) show that the crystal structure of monoclinic α-PaBr₅ is probably not correct.