A Revised Structure Model for the UCl 6 Structure Type, Novel Modifications of UCl 6 and UBr 5 , and a Comment on the Modifications of Protactinium Pentabromides

The redetermination of the crystal structure of trigonal UCl 6 , which is the eponym for the UCl 6 structure type, showed that certain atomic coordinates had been incorrectly reported. This led to noticeably different U−Cl distances within the octahedral UCl 6 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‐UCl 6 , crystallizes in the trigonal space group P 3 ‾ m 1, No. 164, hP 21, with a= 10.907(2), c= 5.9883(12) Å, V= 616.9(2) Å 3 , Z= 3 at T= 253 K. A new low‐temperature (lt) modification of UCl 6 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‐UCl 6 crystallizes isotypic to a low‐temperature modification of SF 6 in the monoclinic crystal system, space group C 2/ m , No. 12, mS 42, with a= 17.847(4), b= 10.8347(18), c= 6.2670(17) Å, β =96.68(2)°, V= 1203.6(5) Å 3 , 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 UBr 5 a new modification was obtained that crystallizes in the triclinic crystal system, space group P 1 ‾ , No. 2, aP 36, 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) Å 3 , Z= 3 at T= 100 K. In this structure the UBr 5 units are dimerized to U 2 Br 10 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 MX 5 ( M =Pa, U; X =Cl, Br) show that the crystal structure of monoclinic α‐PaBr 5 is probably not correct.