I’m afraid I disagree. Why would the molecule choose to be bent? It can be sp hybridized and linear, placing the positive charge in a p-orbital on carbon, and making the C-R sigma bond lower in energy since it has more s-character, or it can be sp2 hybridized with the charge in an sp2 orbital. It seems to me there would be a steric AND electronic penalty for the acylium cation to be anything but linear.
]]>Hi Jonathan!
You posed a very interesting question. Allow me to contribute to the debate.
The linear resonance structure of the acylium cation is an oxonium cation: the positive charge is localized in the oxygen atom. Oxygen is very electronegative and more electronegative than carbon. Oxonium cations are thus hardly stable (although there are exemples in nature – anthocyanins) and consequently the contribution of this resonance structure to the resonance hydrid should be small.
I would expect the resonance hybride to have a slightly larger angle than the typical 120 degrees for sp2 carbons.