Conjugation in the Series of Phosphorus(III) Compounds

In its electronic effect phosphorus(III) is usually regarded as an analogue of tervalent nitrogen. However, in recent years it has been shown that substituents with phosphorus(III) do not exhibit a π-donor effect, which is characteristic to a greater or smaller extent of groups containing the elements in Periods II and III with lone pairs of electrons. On the other hand, substituents with phosphorus(III) exhibit a π-acceptor effect, characteristic of meta-orienting groups, and probably due to the interaction with the vacant orbitals of phosphorus. Thus, in contrast to elements of Periods II and III with lone electron pairs, phosphorus(III) is incapable of p–π conjugation. The absence of the capacity of the lone electron pair of phosphorus to undergo delocalisation (or the presence of only insignificant capacity of this kind) has been explained by an enhanced s-character of the lone pair orbital, which is confirmed by data for the valence angles in phosphorus(III) compounds. The physical and chemical properties of phosphorus(III) compounds in which there is a possibility of p–π conjugation are discussed in the review from this standpoint. All the properties are consistent with the hypothesis of the predominant s-character of the lone electron pair of phosphorus.
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