Molecular structures of the sixth period metal pentachlorides, MCl₅ (M = Ta, W or Re), determined by gas electron diffraction; is Jahn-Teller distortion of WCl₅ quenched by spin-orbit coupling?

The molecular structure of TaCl₅ has been optimised under D_3h symmetry by density-functional theory calculations. Calculation of the molecular force field and vibrational frequencies showed that the optimised structure corresponds to a minimum on the full potential energy surface. Gas electron diffraction data of MCl₅ (M = Ta, W or Re), recorded with nozzle temperatures ranging from 130 to 210°C, showed that WCl₅ and ReCl₅ are trigonal bipyramidal like TaCl₅. Structure refinements based on molecular models of D_3h symmetry lead to satisfactory agreement between experimental and calculated intensities for each compound and yield the M-Cl bond distances (ax/eq): Ta 231.3(5)/226.6(4); W 229.1(4)/224.1(5); Re 226.2(12)7223.7(7) pm. Tungsten pentachloride is a d¹ compound and might have exhibited dynamic or static Jahn-Teller distortion from D_3h symmetry; it has been suggested that such distortion is quenched by strong spin-orbit coupling.