Ylides, definition phosphonium

An examination of the Jc-p coupling constants (Table 3) reveals a large difference between values for ylides (100-130 Hz) and the corresponding salts (47-59 Hz) in all cases (26a). These values are also different from Jc-p for the P—Cphenyi bond, which is approximately the same in both the ylides and the salts (86-92 Hz) (26b). Although the C data provide no definitive answers regarding the nature of the ylide bond, these data indicate that ylides cannot be described as typical delocalized sp hybridized carbanions nor can they be viewed simply as localized sp hybridized carbanions. This information points instead to a substantially localized carbanion of intermediate hybridization, stabilized by an adjacent phosphonium center (26-28). Theoretical considerations have reached similar conclusions (29). 

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