The preference of many solid mercury compounds for "molecular" structures with lower characteristic coordination numbers (frequently CN = 2) and lower boiling points than the corresponding zinc or cadmium species is due to relativistic effects. In particular, the relativistic increase of the mercury 6s-orbital ionization energies reduces the charge separation in and the intermolecular interactions between HgX2 Molecules containing electronegative substituents X. These are the major conclusions of extensive quasirelativistic and nonrelativistic ab initio pseudopotential Hartree-Fock and MP2 calculations on the dimeric systems (HgX2)2 (X = F, Cl, Br, I, H) and on the HgX2 monomers. While quasirelativistic pseudopotential structure optimizations lead to weakly associated C2h complexes of two almost linear HgX2 units with Hg-X distances that are similar to those in the corresponding HgX2 solid-state structures, use of a nonrelativistic Hg pseudopotential results in symmetrically bridged D2h structures with far larger dimerization energies. Only (HgH2)2 exhibits slightly unsymmetrical bridging even with the nonrelativistic Hg pseudopotential. Natural population analyses (NPA) and the electron localization function (ELF) have been employed to rationalize the computed structural and thermochemical trends. While traditional explanations involving sd- or sp-hybridization arguments may have some bearing on the structures of HgH2 or of organomercury compounds, electrostatic interactions and their relativistic reduction seem to be more important for the structural chemistry of mercury dihalides and similar compounds with electronegative ligands.