Phosphide anions

Stable compounds with phosphide anions are represented by Li (12-crown-4)+ PPhj, Et4N+ PlCFj) and K (18 crown)+ P (CN)2 (see 8.51 and 6.448a,b,d). 

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Preparation of Phosphines from Metallated Phosphines.- It has been shown that phosphide anions may be generated from primary and secondary phosphines under aqueous conditions using concentrated aqueous alkali in DMSO or other dipolar aprotic solvents. 

Trofimov, B.A., Gusarova, N.K., and Brandsma, L., Generation of phosphide anions from phosphorus red and phosphine in strongly basic systems to form organylphosphines and -oxides, Phosph., Sulf. Silic., 109-110, 610, 1996. 

Phosphide anions, phosphinyl radicals, and phospheniutn cations are two-coordinate phosphorus species which are related by gain or loss of one electron ... 

Phosphide anions are well documented and are used widely as reagents. However, much less is known concerning phosphinyl radicals and phosphenium cations. 

Interesting work by Lappert, Goldwhite, and co-workers (1,2) has established that phosphinyl radicals can be stabilized by means of bulky substituents such as (Me3Si)2CH. We have found that these radicals constitute a new class of ligand intermediate in character between phosphenium cations and phosphide anions. 

Almost all metals form phosphides, and over 200 different binary compounds are now known. In addition, there are many ternary mixed-metal phosphides. These phosphides consist of metal cations and phosphide anions. In addition to some simple anions (P3-, P -, P ), there are many polyphosphide anions that exist in the form of rings, cages, and chains, as shown in Fig. 15.3.4. 

In some cases, the phosphide anions produced in reactions also contain hydrogen atoms boimd to the structure. Thus, a recent example has shown that direct reduction of white phosphorus with sodium in liquid ammonia generates the catena-trihydrogen triphosphide... 

The generation of phosphine (and the phosphide anion) from elemental phosphorus for use in this type of reaction is common. While unreactive with neutral water (although undergoing rapid oxidation with oxygen at 34 °C), elemental phosphorus (white, P4) generates phosphine readily in basic medium. On heating above 400 °C, white phosphorus [P4] generates a different allotropic form known as red phosphorus in which a P-P bond within the fundamental P4 tetrahedron is broken and individual tetrahedra are linked in a polymeric chain. This form of elemental phosphorus is less reactive that white phosphorus, but still is quite useful for synthetic purposes. 

Two other types of results equally suggest a strong electronic delocalization within the phospholyl anions. First, Quin has found that the NMR signals of these anions are located at much lower field than those of more classical phosphide anions. The available data from Quin and from our own laboratory are collected in Table 15. 

A second striking observation was made by Quin phospholyl anions do not react with absolute ethanol in contrast to classical phosphide anions. This indicates a much reduced basicity for the phospholyl species, once again in agreement with a strong aromaticity. 

The formation of secondary phosphide anions by metallation of secondary phosphines with butyllithium is an extremely efficient practical route for the introduction of the -PPh2 group, and numerous examples are cited in the following sections.50... 

The chemistry of diphosphines and polyphosphines centers on the facile cleavage of the P—P bond (by alkali metals, organometallic reagents), affording phosphide anions. From these,... 

Further instances of the probable attack of phosphide anions on halogen have appeared. Lithium bis(trimethylsilyl)phosphide reacts with 1,2-dibromoethane to form the diphosphine (17), together with ethylene. Similarly, the reaction of... 

Preparation of Phosphines from Metallated Phosphines. - Alkali metal-free phosphide anions have been shown to be formed under synthetically useful conditions in the equilibria between primary or secondary phosphines with the Schwesinger bases (23). Techniques have been developed for the preparation of alkali metal diphenylphosphide reagents in high purity, as evidenced by P nmr studies. The same paper reports a study of the course of the reactions between potassium diphenylphosphide and a series of aryl-, n-alkyl- and neopentyl-halides. The results provide the first evidence of the involvement of single electron transfer (SET) processes in the reactions of alkyl halides. This pathway is dominant in the case of neopentyl-type iodides, but plays only a minor role in the related reactions of neopentyl-type bromides and chlorides. No evidence was adduced of the involvement of SET processes in the reactions of unhindered... 

Phosphide anions, i 2P are also known to function as bridging ligands/ In this regard, they resemble the more familiar bridging halide and chalconide ligands. We have, somewhat arbitrarily, excluded these from this review. 

In trigonal bipyramidal compounds such as PCI5, an outer shell of 10 electrons is involved (3.9a), while in octahedral configurations such as PFg, a negative charge is acquired and the outer shell probably contains 12 electrons (3.9b). The phosphide anion P probably exists in some metal phosphides and this will presumably be based on a completed octet of electrons (3.9c). A similar situation occurs in the phosphide PHj anion (3.9d). 

The phosphide anion, PHj, is obtained from phosphine by reaction with an amide. 

FIGURE 4.16 The phosphide anion HP, f. (From E.J. Lowe, Mellor s Comprehensive Treatise on Inorganic and Theoretical Chemistry, Vol. 8, Snpp. 3, Longmans, London, 1971 Reprinted from Phosphorus 2000 —, D.E.C. Corhridge, p. 123, Elsevier, Amsterdam, 2000. Copyright 2000, with permission from Elsevier.)... 

Only in the 1960s did and compounds become established. The comparatively rare 2-connected phosphanes are represented by h d phosphenium cations (6.5a) and phosphide anions (6.5b), while and compounds are represented by phosphorus pentaphenyl (6.5c) and the hexaphosphoride anion (6.5d). 

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