The Perfluoro-o-phenylene-mercury Trimer [Hg(o-C₆F₄)]₃ - a Textbook Example of Phase-Dependent Structural Differences

The geometric and electronic structure of [Hg(o-C₆F₄)]₃ (1) in the gas phase, i. e. free of intermolecular interactions, was determined by a synchronous gas-phase electron diffraction/mass spectrometry experiment (GED/MS), complemented by quantum chemical calculations. 1 is stable up to 498 K and the gas phase contains a single molecular form: the trimer [Hg(o-C₆F₄)]₃. It has a planar structure of D_3h symmetry with a Hg-C distance of 2.075(5) Å and a Hg-Hg distance of 3.614(7) Å (both r_h1). Structural differences between the crystalline and gaseous state have been analyzed. Different DFT functional-basis combinations were tested, demonstrating the importance to consider the relativistic effects of the mercury atoms. The combination PBE0/MWB(Hg),cc-pVTZ(C,F) turned out to be the most appropriate for the geometry optimization of such organomercurials. The electronic structure of 1, the nature of the chemical bonding in C-Hg-C fragments and the nature of the Hg⋅⋅⋅Hg interactions have been analyzed in terms of the Natural Bond Orbital (NBO) and Quantum Theory of Atoms in Molecules (QTAIM) approaches. The influence of the nature of halogen substitution on the structure of the molecules in the series [Hg(o-C₆H₄)]₃, [Hg(o-C₆F₄)]₃, [Hg(o-C₆Cl₄)]₃, [Hg(o-C₆Br₄)]₃ was also analyzed.