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

The redetermination of the crystal structure of trigonal UCl , which is the eponym for the UCl structure type, showed that certain atomic coordinates had been incorrectly reported. This led to noticeably different U-Cl distances within the octahedral UCl 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 , 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) Å , Z=3 at T=253 K. A new low-temperature (lt) modification of UCl is also presented that was obtained by cooling a single crystal of rt-UCl The phase change occurs between 150 and 175 K. lt-UCl crystallizes isotypic to a low-temperature modification of SF 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) Å , 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 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) Å , Z=3 at T=100 K. In this structure the UBr units are dimerized to U Br 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 (M=Pa, U; X=Cl, Br) show that the crystal structure of monoclinic α-PaBr is probably not correct.