Pπ-dπ Bonding. Ylids - Delocalized Chemical Bonding - Introduction - March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 7th Edition (2013)

March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 7th Edition (2013)

Part I. Introduction

Chapter 2. Delocalized Chemical Bonding

2.H. Pπ–dπ Bonding. Ylids

In Section 1.D, it was stated that, in general, atoms of the second row of the periodic table do not form stable double bonds of the type discussed in Chapter 1 (π bonds formed by overlap of parallel p orbitals). However, there is another type of double bond that is particularly common for the second-row atoms sulfur and phosphorus. Such a double bond is found in sulfurous acid, (H2SO3). While the S=O double bond contains one s orbital, the second orbital is not a π orbital formed by overlap of half-filled p orbitals. Instead it is formed by overlap of a filled p orbital from the oxygen with an empty d orbital from the sulfur. It is called a pπ–dπ orbital.91 Note that this molecule may be represented by two canonical forms, but the bond is nevertheless localized, despite the resonance that is inherent to this structure. Some other examples of pπ–dπ bonding are the phosphine oxides, sulfones, hypophosphorus acid, and sulfoxides. Nitrogen analogues are known, but they are less stable than the phosphorus compounds because the resonance is lacking. For example, amine oxides, analogues of phosphine oxides, can only be written R3N+–O. The pπ–dπ canonical form is impossible since nitrogen is limited to eight outer-shell electrons.

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In all the examples given above, an oxygen atom donates the electron pair and, indeed, oxygen is the most common such atom. In another important class of compounds, called ylids, this atom is carbon.92 There are three common types of ylids: P,93 S,94 and N ylids,95 although As,96 Se, and so on, ylids are also known. Ylids may be defined as compounds in which a positively charged atom from group 15 or 16 of the periodic table is connected to a carbon atom carrying an unshared pair of electrons (+ and – charges on adjacent atoms). Because of pπ–dπ bonding, two canonical forms can be written for P and S, but there is only one for N ylids. Phosphorus ylids are much more stable than N ylids (see also 12–22) which is one reason why N ylids tend to react more like carbanions (see 13–31 and 18–21). While S ylids are generally less stable than P ylids, they are rather common.

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In almost all compounds that have pπ-dπ bonds, the central atom is connected to four or three atoms and an unshared pair, and the bonding is approximately tetrahedral. The pπ–dπ bond, therefore, does not greatly change the geometry of the molecule in contrast to the normal π bond, which changes an atom from tetrahedral to trigonal. Calculations show that nonstabilized phosphonium ylids have nonplanar ylid carbon geometries whereas stabilized ylids have planar ylid carbons.97