Lewis bases, phosphorus compounds acting

Adduct formation results in well-defined species. Generally speaking, phosphorus compounds act as Lewis bases [exceptions being penta-valent phosphorus halides as reviewed by Webster <1966 106)] for other examples in which the relevant Lewis acids are metalloid derivatives see references 1966, 107 and 1969, 186. Adducts involving boron have recently been reviewed elsewhere(1969 94 andl02) and are by far the most numerous and use has been made of phosphorus, boron, proton and fluorine resonances, in some cases at varying temperature. 

In general, P(III) compounds adopt the geometry of a trigonal pyramid with the free electron pair on the phosphorus [32]. This property is responsible for the fact that trivalent phosphorus compounds are Lewis bases and can act as soft nucleophiles preferentially in the reaction with soft electrophiles. Upon protonation of the phosphorus, it can also behave as an electrophile. The same situation can be caused by strong electron-withdrawing substituents. 

Compounds containing carbenium, silyl or phosphonium cations can act as Lewis acids. In addition, phosphorus- and silicon-based hypervalent compounds display a Lewis acid catalytic activity. Furthermore, ionic liquids, organic salts with a melting point below 100 °C, have shown the ability to catalyze a group of reactions either in substoichiometric amount or, if used as the reaction medium, in stoichiometric or even larger quantities. The solvents can be efficiently recovered after the reaction. Each type of these compounds will be discussed in a separate section. 

The rapid evolution and the multifarious activity in the field of phosphorus chemistry is well illustrated by the proposal (Dupart)332 of a new systematic classification of free and coordinated phosphorus compounds to complete the systems proposed by Wolf333 and by Perkins et a/.334. In this new system, a code number NFE(NPL)D could be used to describe the compound, in which NFE would be the number of free electrons, N the number of valence shell electrons, L the number of ligands and D the number of electron doublets donated or accepted, which would be negative if the P atom acted as a donor (a Lewis base), or positive if P acted as an acceptor (a Lewis acid). 

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