A new twist on an old molecule: a rotameric isomer of bis(pentafluorophenyl)mercury

Suspending crystals of bis(pentafluorophenyl)mercury [Hg(C6F5)2] (1a) in a saturated n-hexane solution for a prolonged period results in transformation into the new polymorph [Hg(C6F5)2] (1 b) monoclinic P21/n, a = 11.7060(3) Å, b = 7.8531(2) Å, c = 13.5429(4) Å, β = 107.247(3)°, V = 1189.00(6) Å3. The C-Hg-C angle in 1 b is 177.10(11)° and the pentafluorophenyl rings are rotated 74.6(1)° to each other in comparison to 1a, the form known for 50+ years, which has a rotation angle of 58.0(2)°. Polymorph 1 b could also be obtained by crystallization from chloroform, or a CHCl3/n-hexane (1:1) mixture at −20 °C, or by seeding with a crystal of 1 b. Theoretical calculations suggest that the barrier to rotation is low