°C. Bond lengths deduced from X-ray crystallography are consistent with a cyclohexadienyl cation structure. Acylium ions An acylium ion is a cation with the formula RCO+. The structure is described as R−C≡O+ or R−=O. It is the synthetic and reactive equivalent of an acyl carbocation, but the actual structure has the oxygen and carbon linked by a triple bond. Such species are common reactive intermediates, for example, in the Friedel−Crafts acylations also in many other organic reactions such as the Hayashi rearrangement. Salts containing acylium ions can be generated by removal of the halide from acyl halides: RCOCl + SbCl5 → RCO+ The C–O distance in these cations is near 1.1 ångströms, even shorter than that in carbon monoxide. Acylium cations are characteristic fragments observed in EI-mass spectra of ketones. See also Borenium ion Nitrenium ion References Cations Reactive intermediates Carbocations