Bonding in Phosphonium Ylides

Bruckner R (author), Harmata M (editor) In Organic Mechanisms - Reactions, Stereochemistry and Synthesis Chapter DOI 10.1007/978-3-642-03651-4 ll, Springer-Verlag Berlin Heidelberg 2010 

11 Conversion of Phosphorus- or Sulfur-Stabilized C Nucleophiles with Carbonyl Compounds 

However, it seems more likely that the formal charge at the carbanionic center of these ylides is rather stabilized to a considerable extent by two other effects  

stabilization of the electron pair on the carbanionic carbon atom by an anomeric effect. 

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Structure and bonding in phosphonium ylides, salts and phosphoranes... 

For reasons similar to those for the oxides2, studies of the bonding in phosphonium ylides have concentrated on the nature of the P=C bond regarding the following issues (i) the strength of the bond, although in reality this has meant its reactivity (see below) (ii) the exact distribution of the electron density with discussions very similar to those for the oxides and (ii) the difference between phosphorus, nitrogen and (to a lesser extent) arsenic ylides. 

The structural analyses in this chapter present a fairly unihed description of the bonding in phosphonium ylides. The bonding in these compounds can best be described by the ylide resonance form A with a small contribution from the ylene form B. The evidence for this conclusion is summarized below. 

Mechanistic and Theoretical Studies of Phosphonium Ylides and the Wittig Reaction. - The physico-chemical nature of the P-C bond in phosphonium ylides is complex and, despite intensive research over many years, remains the subject of dispute Numerous theoretical studies of this problem have appeared in the scientific literature. A recent contribution to this area by Mitrasinovic uses sharing indices and sharing amplitudes to study P-C bonds in a number of tri-and penta-valent phosphorus species. Sharing indices and amplitudes are quantitative, orbital dependent, measurements of the degree to which an electron, as a wave, is shared between two spatial points in a many electron system. Ylides studied using this method include (1), (2) and (3). ... 

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