Ionic Phosphides

FIGURE 8.5 Structures of cubic monophosphides (a) AlP (zinc blende type), (b) SnP (rocksalt type). In each structure, both kinds of atoms have similar environments. 

The existence of polymerised phosphide anions (8.14) is now firmly established. Both the monophosphides CaP and SrP consist of an assembly of cations and anions. On contact with water they liberate diphosphine (Section 4.4), which, like the anion contains a P-P linkage (8.15). Both P3- and 2 anions appear to be present in the crystal structures of Ba4P3, Li4Eu3P4 and CojRejPij. 

Pure calcium monophosphide, CaP, is best prepared by passing phosphine into calcium dispersed in liquid ammonia. The addition compound which is formed can then be heated at 150°C to give the monophosphide, which above 600°C converts into a lower phosphide with loss of phosphorus (8.16). The lower phosphide liberates phosphine on contact with water (8.17). It is made commercially by heating quicklime in phosphorus vapour, and, together with Mg3P2, it finds application in sea flares. Sea flares are spontaneously inflammable due to the presence of small quantities of diphosphine which arise from the presence of impurity CaP (8.15). 

Both P and P2 anions are present in the crystal structures of Ba4P3, Li4Eu3P4, Ba InPn and Co2Re5Pi2. The P anion probably exists in complexes such as (8.244a). 

Reaction (8.15) can be compared with the action of water on calcium carbide which liberates acetylene, HC CH. Zinc phosphide, Zn3P2, has a long-established use as a rodenticide. 

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In contrast to ionic phosphides which upon protonolysis yield P2H4 along with PHj (see 1.5.3.2.1), ionic nitride protonolyses do not yield N H, except for BajN which, when heated with H O at 380°C, produces N H. ... 

In addition, phosphides of the electropositive elements in Groups 1, 2 and the lanthanoids form phosphides with some degree of ionic bonding. The compounds NasPn and SrsP have already been mentioned (p. 484) and other somewhat ionic phosphides are M3P (M = Li, Na), M3P2 (M = Be, Mg, Zn, Cd), MP (M = La, Ce) and Th3P4. However, it would be misleading to consider these as fully ionized compounds of P " and there is extensive metallic or covalent interaction in the solids. Such compounds are characterized by their ready hydrolysis by water or dilute acid to give PH3. 

Although we all feel at home with ammonia and recognize its distinctive pungent smell, phosphine is a different kettle of fish. In fact, that is exactly what it smells like—rotten fish. It is conveniently prepared by the reaction of water and an ionic phosphide like that of calcium, as shown in Equation (16.2). In the open air, the deadly poisonous phosphine gas immediately reacts with oxygen (and ignites due to traces of P2H4 or elemental phosphorus), as shown in Equation (16.3) ... 

The term S] is defined by the authors as an ionicity factor and assumes a value of 0.5 for oxides and silicates, 0.75 for halides, 0.40 for calcogenides, 0.25 for phosphides and arsenides, and 0.2 for nitrides and carbides (Z is the anion charge). Equation 1.94 is based on the thermal expansion data listed in table 1.15. 

If we examine the distances listed in Table 7.2 some interesting facts emerge. For a given metal A. the A—P distance is constant as we might expect for an ionic alkaline earth metal-phosphide bond. Furthermore, these distances increase calcium < strontium < barium in increments of about 15 pm os do the ionic radii of Ca2+. St7, and Ba- (Table 4.4). However, the B—P distances vary somewhat more with no periodic trends (Mn. Cu larger Ni, Fe, Co smaller). Most interesting, however, is the huger variability in the P—P distance from about 380 pm (Mn. Fe) to 225 pm (Cu). As it Luros Out, the lower limit of 225 pm (Cu) is a typical value for a P— P bond (Table E.l,... 

D.12 Write formulas for the ionic compounds formed from (a) calcium and chloride ions (b) iron(III) and sulfate ions (c) ammonium and iodide ions (d) lithium and sulfide ions (e) calcium and phosphide ions. 

Even though phosphorus has a lower electronegativity than nitrogen, it will form phosphides (ionic substances containing the P3- anion) such as Na3P and Ca3P2- Phosphide salts react vigorously with water to produce phosphine (PH3), a toxic, colorless gas ... 

Recall from Section 8.2 that a sample of an ionic compound contains crystals formed from many ions arranged in a pattern. Because no single particle of an ionic compound exists, ionic compounds are represented by a formula that provides the simplest ratio of the ions involved. The simplest ratio of the ions represented in an ionic compound is called a formula unit. For example, the formula KBr represents a formula unit for potassium bromide because potassium and bromide ions are in a one-to-one ratio in the compound. A formula unit of magnesium chloride is MgCl2 because two chloride ions exist for each magnesium ion in the compound. In the compound sodium phosphide, three sodium ions exist for every phosphide ion. What is the formula unit for sodium phosphide ... 

Inhalation of gas (rare) or ingestion of solid phosphide reacts with gastric HC1 to form phosphine gas, which interferes with enzymes and protein synthesis, primarily in mitochondria of heart and lung cells, causing myofibril necrosis and secondary ionic changes in peripheral small vessels and lungs... 

The nitrides (and phosphides) of many of the transition metals form crystals with the sodium chloride structure. This description, however, must not be interpreted as implying that they are ionic in character, for in fact they display many of the properties of intermetallic systems. For this reason a discussion of these nitrides is deferred to chapter 13. 

Many carbides and silicides of composition AX are formed by transition metals. These carbides and silicides are characterized by very high melting points, extreme hardness, optical opacity and relatively high electrical conductivity. Many of them have the sodium chloride structure but they are not ionic compounds rather do they resemble the corresponding nitrides and phosphides in simulating alloy systems in many of their properties. For this reason they will be discussed later. 

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