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

Abstract

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 Ptrue3‾ m1, No. 164, hP21, with a=10.907(2), c=5.9883(12) Å, V=616.9(2) Å3, 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‐UCl6. 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) Å3, 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 Ptrue1‾ , 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) Å3, 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.

Complexes featuring a linear [N≡U≡N] core isoelectronic to the uranyl cation

“…47 UBr5 was prepared from the elements. 27,48 The purity of both compounds was checked with powder X-ray diffraction, IR and Raman spectroscopy (Supplementary Figures 1-4). Single-crystal X-ray analyses: Single crystals were extracted from liquid ammonia at -78 °C, selected under exclusion of air in cooled perfluorinated polyether (Galden LS 230, Solvay Solexis) and mounted using the MiTeGen MicroLoop system.…”

Rerefinement of the crystal structure of trichloridosulfonium(IV) hexachloridouranate(V), (SCl3)[UCl6]

“…data reports U-Cl distances are observed: (i) in the crystal structure of the low-temperature modification of UCl 6 where the coordination sphere for the U atom is also distorted octahedral but with slightly shorter U-Cl bonds of 2.4443 (15)-2.4570 (20) Å (at 100 K; Deubner et al, 2019) due to the presence of a U VI atom, and (ii) in Cs 2 [UCl 6 ] with longer U-Cl bonds of 2.621 Å (at 293 K; Schleid et al, 1987) due to the presence of an U IV atom.…”