Dication

A dication is any cation, of general formula X2+, formed by the removal of two electrons from a neutral species.

Diatomic dications corresponding to stable neutral species (e.g. formed by removal of two electrons from H2) often decay quickly into two singly charged particles (H+), due to the loss of electrons in bonding molecular orbitals. Energy levels of diatomic dications can be studied with good resolution by measuring the yield of pairs of zero-kinetic-energy electrons from double photoionization of a molecule as a function of the photoionizing wavelength (threshold photoelectrons coincidence spectroscopy – TPEsCO). The dication is kinetically stable.

An example of a stable diatomic dication which is not formed by oxidation of a neutral diatomic molecule is the dimercury dication. An example of a polyatomic dication is, formed by oxidation of S8 and unstable with respect to further oxidation over time to form SO2.

Many organic dications can be detected in mass spectrometry for example (a  complex) and the acetylene dication. The adamantyl dication has been synthesized.

Divalent metals
Some metals are commonly found in the form of dications when in the form of salts, or dissolved in water. Examples include the alkaline earth metals (Be2+, Mg2+, Ca2+, Sr2+, Ba2+, Ra2+); later 3d transition metals (V2+, Cr2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+); group 12 elements (Zn2+, Cd2+, Hg2+); and the heavy members of the carbon group (Sn2+, Pb2+).

Presence in space
Multiply-charged atoms are quite common in the Solar system in the so-called Solar wind. Among these, the most abundant dication is He2+. However, molecular dications, in particular CO22+, have never been observed so far though predicted to be present for instance at Mars. Indeed, this ion by means of its symmetry and strong double bounds is more stable (longer lifetime) than other dications. In 2020, the molecular dication CO22+ has been confirmed to be present in the atmosphere of Mars and around Comet 67P.