Mercury is unique among the group 12 metals for its ability to form the M–M bond so readily.
 Hg₂Cl₂ is a linear molecule.
 The mineral calomel crystallizes in the tetragonal system, with space group I4/m 2/m 2/m.
 The unit cell of the crystal structure is shown below:
The Hg–Hg bond length of 253 pm (Hg–Hg in the metal is 300 pm) and the Hg–Cl bond length in the linear Hg₂Cl₂ unit is 243 pm.
 The overall coordination of each Hg atom is octahedral as, in addition to the two nearest neighbours, there are four other Cl atoms at 321 pm.
Longer mercury polycations exist.
Mercury is unique among the group 12 metals for its ability to form the M–M bond so readily.  Hg₂Cl₂ is a linear molecule.  The mineral calomel crystallizes in the tetragonal system, with space group I4/m 2/m 2/m.  The unit cell of the crystal structure is shown below:
The Hg–Hg bond length of 253 pm (Hg–Hg in the metal is 300 pm) and the Hg–Cl bond length in the linear Hg₂Cl₂ unit is 243 pm.  The overall coordination of each Hg atom is octahedral as, in addition to the two nearest neighbours, there are four other Cl atoms at 321 pm. Longer mercury polycations exist.
Mercury(I) chloride