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

Area of Science:

• Organometallic Chemistry
• Physical Chemistry
• Computational Chemistry

Background:

• Organomercurial compounds exhibit unique structural and electronic properties.
• Understanding the gas-phase behavior of organomercurials is crucial for elucidating their intrinsic properties, free from intermolecular interactions.
• The trimer [Hg(o-C₆F₄)]₃ is a relevant model system for studying mercury-carbon and mercury-mercury interactions.

Purpose of the Study:

• To determine the geometric and electronic structure of [Hg(o-C₆F₄)]₃ in the gas phase.
• To investigate the nature of chemical bonding within the C-Hg-C fragments and Hg⋅⋅⋅Hg interactions.
• To compare the gas-phase structure with its crystalline form and analyze the influence of halogen substitution on related molecules.

Main Methods:

• Synchronous gas-phase electron diffraction/mass spectrometry (GED/MS) experiment.
• Quantum chemical calculations, including Density Functional Theory (DFT) with various functional-basis combinations.
• Relativistic effects of mercury atoms were considered.
• Natural Bond Orbital (NBO) and Quantum Theory of Atoms in Molecules (QTAIM) analyses were employed.

Main Results:

• The gas phase of [Hg(o-C₆F₄)]₃ at 498 K contains a single molecular form: the planar D_3h trimer.
• Determined bond distances: Hg-C at 2.075(5) Å and Hg-Hg at 3.614(7) Å (r_h1).
• The PBE0/MWB(Hg),cc-pVTZ(C,F) combination was found to be most appropriate for geometry optimization, highlighting the importance of relativistic effects.

Conclusions:

• The gas-phase structure of [Hg(o-C₆F₄)]₃ is a planar D_3h trimer.
• NBO and QTAIM analyses provided insights into the C-Hg-C bonding and Hg⋅⋅⋅Hg interactions.
• Structural differences between gas and crystalline phases were observed, and the impact of halogen substitution was systematically analyzed for a series of related compounds.