A new type of “push-pull” cyclobutadiene is encountered in the resonance formula (1) of the dimers of {article}{amssymb}{empty}{document}${ Ph}_3 { P}^ ⊕ 1pt--- { C}^ ⊖ 1pt=-3.45pt= { C} 1pt=-3.45pt= { X}, { X}1pt=-3.45pt={ O}${document} or NPh. The dimers form yellow crystals which melt with decomposition at 278–280 or 178°C, respectively.
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Bestmann et al. (1977) studied this question.
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