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

DFT calculations (M06‐2X, B97D3, and MP2) indicate that polar covalent bonding in (HgF2)n begins at n = 5.

Evidence is provided for a transition from weak noncovalent interactions among HgF2 monomers to stronger polar covalent interactions in mercury difluoride clusters, (HgF2)n, as the size of the cluster (the value of n) increases. This transition—the so‐called “ionic switch” that was predicted in an earlier article in this journal—is explained as a triumph of electrostatic interactions (which favor short and strong bonds) against the rigidity imposed on mercury dihalide molecules by relativistic effects in Hg. The observations afford some new insights into nucleation processes. The complete account can be found in the Full paper by K. J. Donald and colleagues on