= Acylium ion
{title2=$\mathrm{RCO^+}$}
An <acylium ion> is a positively charged acyl species, commonly written $\mathrm{RCO^+}$, with charge and bonding described by resonance forms $\mathrm{R-C\equiv O^+}$ and $\mathrm{R-C^+=O}$. Its carbon is strongly electrophilic.
Acylium-like species explain carbonyl-group transfer in <Friedel-Crafts acylation>. Unlike an ordinary alkyl <carbocation>, the acylium ion is stabilized by oxygen donation and usually does not undergo the same skeletal rearrangements. The precise reactive species can include a counterion or Lewis-acid complex, so the isolated ion drawing is a mechanistic model.
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