Double bonds major contributor

The phosphorus ylide has two resonance forms one with a double bond between carbon and phosphorus, and another with charges on carbon and phosphorus. The double-bonded resonance form requires ten electrons in the valence shell of phosphorus, using a d orbital. The pi bond between carbon and phosphorus is weak, and the charged structure is the major contributor. The carbon atom actually bears a partial negative charge, balanced by a corresponding positive charge on phosphorus. 

In the structure of ylides, the ability of sulfur to stabilise an adjacent negative charge is an interesting phenomenon which probably involves the delocalisation of electron density into the low-lying 3d-orbitals of sulfur. Essentially, the question revolves around the relative importance of the resonance forms (2a) and (2b) (Scheme 1) in the ground state structure of sulfur ylides. The evidence tends to indicate that sulfur ylides do not contain appreciable double-bond character, so that the structure (2a) is probably the major resonance contributor, which would be in agreement with the preferred geometry of carbon and sulfur.1... 

Resonance forms may be drawn for (b) and hjfi of Problem 28—the structures containing double bonds. (It is always possible to draw a resonance form for a structure with a multiple bond, although the resonance form you get is not necessarily a major contributor.)... 

There is no consensus regarding which Lewis formula is the major contributor in these and related sulfur-containing compounds. The lUPAC recommends writing double bonds rather than dipolar single bonds that is, B for dimethyl sulfoxide and E for dimethyl sulfone. 

This delocalization, however, produces a contributing structure that has one less bond than the major contributor. Consequently, electron release to double bonds by alkyl substituents should be, and is observed to be, less pronounced than comparable stabilization of carboca-tions and free radicals—species in which the major and minor contributors have the same number of bonds. 

Two resonance forms depict the structure of the ylide. The dipolar resonance form is generally used to show the mechanism of the Wittig reaction. This resonance form has a single bond between the phosphorus and carbon atoms. Although positive and n ative charges are located on adjacent atoms, the dipolar form is the major contributor to the yhde structure. The uncharged resonance form has a double bond between the phosphorus and carbon atoms, so the phosphorus atom has... 

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