dication
{{Short description|Divalent positive ion}}
{{Distinguish|Dictation (disambiguation)}}
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. {{chem|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 {{chem|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 {{chem|Hg|2|2+}}. An example of a polyatomic dication is {{chem|S|8|2+}}, 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 {{chem|CH|4|2+}} (a {{chem|CH|2|2+|·H|2}} complex) and the acetylene dication {{chem|C|2|H|2|2+}}.{{cite journal |last1=Lammertsma |first1=K. |last2=von Ragué Schleyer |first2=P. |last3=Schwarz |first3=H. |date=1989 |title=Organic Dications: Gas Phase Experiments and Theory in Concert |journal=Angew. Chem. Int. Ed. Engl. |volume=28 |issue=10 |pages=1321–1341 |doi=10.1002/anie.198913211}} The adamantyl dication has been synthesized.{{cite journal |last1=Bremer |first1=Matthias |last2=von Ragué Schleyer |first2=Paul |last3=Schötz |first3=Karl |last4=Kausch |first4=Michael |last5=Schindler |first5=Michael |title=Four-Center Two-Electron Bonding in a Tetrahedral Topology. Experimental Realization of Three-Dimensional Homoaromaticity in the 1,3-Dehydro-5,7-adamantanediyl Dication |journal=Angewandte Chemie International Edition in English |date=August 1987 |volume=26 |issue=8 |pages=761–763 |doi=10.1002/anie.198707611 |url=https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.198707611 |language=en |issn=0570-0833}}
Divalent metals
{{Periodic table (micro)
| title=Common element dications
| mark=He, Be, Mg, Ca, Sr, Ba, Ra, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, Hg, Sn, Pb
}}
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.{{cite journal |last1=Witasse|first1=O. |last2=Dutuit |first2=O. |last3 = Lilensten | first3= J. |last4 = Thissen |first4 = R. |last5 = Zabka | first5= J. | last6= Alcaraz | first6 = C. |last7 =Blelly | first7 = P.-L. |last8 = Bougher | first8 = S. W. | last9 = Engel | first9 = S. | last10 =Andersen | first10 = L.H. |last11 = Seiersen | first11 = K. |date=2020 | title=Prediction of a CO22+ layer in the atmosphere of Mars |journal=Geophysical Research Letters |volume=29 |issue=8 |pages=104-1-104-4 |doi=10.1029/2002GL014781|s2cid=129035959 |doi-access= free }} 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{{cite journal |last1=Gu|first1=H. |last2=Cui |first2=J. |last3 = Diu | first3= D. |last4 = Dai |first4 = L. |last5 = Huang | first5= J. | last6= Wu | first6 = X. |last7 =Hao | first7 = Y. |last8 = Wei | first8 = Y. |date=2020 | title=Observation of CO22+ dication in the dayside Martian upper atmosphere |journal=Earth and Planetary Physics |volume=4 |issue=4 |pages=396–402 |doi=10.26464/epp2020036|s2cid=219929017 |doi-access=free }} and around Comet 67P.{{cite journal |last1=Beth|first1=A. |last2=Altwegg |first2=K. |author2-link=Kathrin Altwegg|last3 = Balsiger | first3= H. |last4 = Berthelier |first4 = J.-J. |last5 = Combi | first5= M. R. | last6= De Keyser | first6 = J. |last7 =Fiethe | first7 = B. |last8 = Fuselier | first8 = S.A. | last9 = Galand | first9 = M. | last10 =Gombosi | first10 = T.I. |last11 = Rubin | first11 = M. | last12 = Sémon | first12= T. |date=2020 | title=ROSINA ion zoo at Comet 67P
|journal=Astronomy and Astrophysics |volume=642 |issue=October 2020 |pages=A27 |doi=10.1051/0004-6361/201936775|s2cid=221172890 |doi-access= free |arxiv= 2008.08430 }}
References
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