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 TaCl 5 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 MCl 5 (M = Ta, W or Re), recorded with nozzle temperatures ranging from 130 to 210 °C, showed that WCl 5 and ReCl 5 are trigonal bipyramidal like TaCl 5 . 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)/223.7(7) pm. Tungsten pentachloride is a d 1 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.