Dication

Not to be confused with Dictation (disambiguation).

A dication is any cation, of general formula X^2+, formed by the removal of two electrons from a neutral species.

Diatomic dications corresponding to stable neutral species (e.g. H^2+
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 He^2+
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 Hg^2+
₂. An example of a polyatomic dication is S^2+
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 CH^2+
₄ (a CH^2+
₂·H
₂ complex) and the acetylene dication C
₂H^2+
₂.[1] The adamantyl dication has been synthesized.[2]

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 (Be^2+, Mg^2+, Ca^2+, Sr^2+, Ba^2+, Ra^2+); later 3d transition metals (V^2+, Cr^2+, Mn^2+, Fe^2+, Co^2+, Ni^2+, Cu^2+, Zn^2+); group 12 elements (Zn^2+, Cd^2+, Hg^2+); and the heavy members of the carbon group (Sn^2+, Pb^2+).

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 He^2+. However, molecular dications, in particular CO₂^2+, 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 CO₂^2+ has been confirmed to be present in the atmosphere of Mars[4] and around Comet 67P.[5]

References

1. Lammertsma, K.; von Ragué Schleyer, P.; Schwarz, H. (1989). "Organic Dications: Gas Phase Experiments and Theory in Concert". Angew. Chem. Int. Ed. Engl. 28 (10): 1321–1341. doi:10.1002/anie.198913211.
2. Bremer, Matthias; von Ragué Schleyer, Paul; Schötz, Karl; Kausch, Michael; Schindler, Michael (August 1987). "Four-Center Two-Electron Bonding in a Tetrahedral Topology. Experimental Realization of Three-Dimensional Homoaromaticity in the 1,3-Dehydro-5,7-adamantanediyl Dication". Angewandte Chemie International Edition in English. 26 (8): 761–763. doi:10.1002/anie.198707611. ISSN 0570-0833.
3. Witasse, O.; Dutuit, O.; Lilensten, J.; Thissen, R.; Zabka, J.; Alcaraz, C.; Blelly, P.-L.; Bougher, S. W.; Engel, S.; Andersen, L.H.; Seiersen, K. (2020). "Prediction of a CO₂^2+ layer in the atmosphere of Mars". Geophysical Research Letters. 29 (8): 104-1 – 104-4. doi:10.1029/2002GL014781. S2CID 129035959.
4. Gu, H.; Cui, J.; Diu, D.; Dai, L.; Huang, J.; Wu, X.; Hao, Y.; Wei, Y. (2020). "Observation of CO₂^2+ dication in the dayside Martian upper atmosphere". Earth and Planetary Physics. 4 (4): 396–402. doi:10.26464/epp2020036. S2CID 219929017.
5. Beth, A.; Altwegg, K.; Balsiger, H.; Berthelier, J.-J.; Combi, M. R.; De Keyser, J.; Fiethe, B.; Fuselier, S.A.; Galand, M.; Gombosi, T.I.; Rubin, M.; Sémon, T. (2020). "ROSINA ion zoo at Comet 67P". Astronomy and Astrophysics. 642 (October 2020): A27. arXiv:2008.08430. doi:10.1051/0004-6361/201936775. S2CID 221172890.