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 D3h symmetry with a Hg–C distance of 2.075(5) Å and a Hg–Hg distance of 3.614(7) Å (both rh1). 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. This study shows how intermolecular interactions in a solid can deform a seemingly rigid molecule. It is one of the rare cases where a series of structural determinations in the crystalline phase and a highly reliable structural determination in the gaseous state by electron diffraction are available. As a heavy element containing structure of a free molecule it can also serve as a reference for the evaluation and development of quantum chemical methods.