Ylene resonance form

The ionic representation of the ylides in Figure 9.1 shows only one of two conceivable resonance forms of such species. In contrast to the N atom in the center of the N ylides, the P or S atoms in the centers of the P and S ylides (Figure 9.1) may exceed their valence electron octets and share a fifth electron pair. For P and S ylides one can therefore also write a resonance form with a C=P or C=S double bond, respectively these are resonance forms free of formal charges (Figure 9.1). For the sulfoxonium ylide there is a second resonance form in which the S atom exceeds its valence electron octet however, this does contain formal charges. Resonance forms of ylides in which the heteroatom exceeds its valence electron octet are called ylene resonance forms. The ene part of the designation ylene refers to the double bond between the heteroatom and the deprotonated alkyl group. 

Nevertheless, it is doubtful as to whether the negative charge on the carbanionic C atom of P and S ylides is indeed stabilized by partial double bond formation to the heteroatom, as is expressed by the ylene resonance form. Probably, the formal charge at the carbanionic center of these ylides is rather stabilized to a considerable extent by two other effects ... 

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. 

It should be remembered that the heteroatom under scrutiny belongs to the second full period of the periodic table of the elements. Thus, the double bond of an ylene form is not a p, p%, but a d%, p% double bond. Though a d%, p% double bond is not very stable, the ylene resonance should make some contribution to the electron distribution in the ylides of Figure 11.1. One reason is that this resonance form must does not suffer from charge separation, as does the ylidic resonance form. 

Phosphorus ylides are carbanions whose negative charge is delocalized by dn-pn- ond ng into the empty 3d orbitals of the phosphorus atom. The resonance forms ylide 5a and ylene or phosphorane 5b are shown in Scheme 2 [17]. 

The stability of phosphorus ylides is ascribed to resonance between the two Lewis structures in which the ylene form implies electron donation into phosphorus 3d-orbitals. Recent careful and nmr spectroscopic studies of trimethyl-... 

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