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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. H2+
2 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 He2+
2 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 Hg2+
2. An example of a polyatomic dication is S2+
8, 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 CH2+
4 (a CH2+
2·H
2 complex) and the acetylene dication C
2H2+
2.[1] The adamantyl dication has been synthesized.[2]
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+).
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.[3] 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[4] and around Comet 67P.[5]
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