Affiliation: CATCO, Department of Chemistry, Southern Methodist University, 3215 Daniel Ave, Dallas, Texas 75275-0314, USA.
The normalized elimination of the small component (NESC) method is a first principles 2-component relativistic approach that leads to the Dirac-exact description of one-electron systems. It is a powerful method to routinely investigate chemical and physical properties of molecules containing relativistic atoms. The vibrational modes of mercury halides are investigated to derive via the corresponding local HgX (X = H, F, Cl, Br, I, At) stretching modes an appropriate measure for the HgX bond strength. It is shown that HgF bonding in HgF4 is stronger than that in HgF2, which is a result of enhanced charge transfer from Hg to the four F atoms and the formation of electron deficient 2e-4c bonds with strong ionic character. A generally applicable bonding model for HgX molecules is outlined.