Acylium ion ($\mathrm{RC}{\mathrm{O}}^{+}$)

An acylium ion is a positively charged acyl species, commonly written $\mathrm{RC}{\mathrm{O}}^{+}$, with charge and bonding described by resonance forms $\mathrm{R}-\mathrm{C}\equiv {\mathrm{O}}^{+}$ and $\mathrm{R}-{\mathrm{C}}^{+}=\mathrm{O}$. 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.