Acylium ions are cations of the formula . The carbon–oxygen bond length in these cations is near 1.1 Å (110-112 pm), which is shorter than the 112.8 pm of carbon monoxide and indicates triple-bond character.
The carbon centres of acylium ions generally have a linear geometry and sp atomic hybridization, and are best represented by a resonance structure bearing a formal positive charge on the oxygen (rather than carbon): . They are characteristic fragments observed in EI-mass spectra of ketones.
Acylium ions are common reactive intermediates, for example in the Friedel–Crafts acylation and many other organic reactions such as the Hayashi rearrangement. Salts containing acylium ions can be generated by removal of the halide from acyl halides:
Acyl radicals are readily generated from aldehydes by hydrogen-atom abstraction. However, they undergo rapid decarbonylation to afford the alkyl radical:
Acyl anions are almost always unstable—usually too unstable to be exploited synthetically. They readily react with the neutral aldehyde to form an acyloin dimer. Hence, synthetic chemists have developed various acyl anion synthetic equivalents, such as dithianes, as surrogates. However, as a partial exception, hindered dialkylformamides (e.g., diisopropylformamide, HCONiPr) can undergo deprotonation at low temperature (−78 °C) with lithium diisopropylamide as the base to form a carbamoyl anion stable at these temperatures.