Ethenium

In chemistry, ethenium, protonated ethylene or ethyl cation is a positive ion with the formula. It can be viewed as a molecule of ethylene with one added proton, or a molecule of ethane  minus one hydride ion. It is a carbocation; more specifically, a nonclassical carbocation.

Preparation
Ethenium has been observed in rarefied gases subjected to radiation. Another preparation method is to react certain proton donors such as trihydrogen cation, helium hydride ion, diazenylium, and with ethane at ambient temperature and pressures below 1 mmHg. (Other donors such as methanium and form ethanium preferably to ethenium.)

At room temperature and in a rarefied methane atmosphere, ethanium slowly dissociates to ethenium and. The reaction is much faster at 90 °C.

Stability and reactions
Contrary to some earlier reports, ethenium was found to be largely unreactive towards neutral methane at ambient temperature and low pressure (on the order of 1 mmHg), even though the reaction yielding sec- and is believed to be exothermic.

Structure
The structure of ethenium's ground state was in dispute for many years, but it was eventually agreed to be a non-classical structure, with the two carbon atoms and one of the hydrogen atoms forming a three-center two-electron bond. Calculations have shown that higher homologues, like the propyl and n-butyl cations also have bridged structures. Generally speaking, bridging appears to be a common means by which 1° alkyl carbocations achieve additional stabilization. Consequently, true 1° carbocations (with a classical structure) may be rare or nonexistent.