The HgF2 Ionic Switch: A Triumph of Electrostatics against Relativistic Odds

A remarkable transition in the chemical bonding in (HgF₂)_n clusters as a function of n is identified and characterized. HgF₂ is a fascinating material. Certain significant consequences of relativistic effects on the structure of the HgF₂ molecule, dimer, and trimer disappear in the extended solid. Relativistic effects in Hg ensure that HgX₂ molecules (X≡F, Cl, Br, and I) are linear, rigid, and form weakly bound dimers and trimers held together by weak electrostatic and van der Waals‐type forces (unlike ZnX₂ and CdX₂ systems in which the intermonomer contacts are strong polar covalent bonds). For HgF₂, the location and nature of an apparent transition from weak interactions in the smallest (HgF₂)_n clusters to ionic bonding in the (fluorite) HgF₂ extended solid has remained a mystery. Computational evidence obtained at the M06‐2X, B97D3, and MP2 levels of theory and reported herein indicate that polar covalent bonding in (HgF₂)_n begins as early as n=5. For n=2 through to n=13, the transition or switch from weak (primarily dipole–dipole‐type) intermonomer interactions to a preference for polar covalent bonding occurs within the range 5<n≤9. Thermodynamic evidence for this transition is provided. Our results demonstrate a significant risk associated with crystal structure prediction from the ground up (i.e., based on bonding patterns in small clusters). The path from monomers through to extended solids may be punctuated at one or several points (as n increases) with transitions in structure and bonding that are not anticipated or betrayed by the bonding in small clusters.