Sulfides and selenides of phosphorus

Sulfur-nitrogen compounds are described in Section 16.10, and in this section we look at the molecular sulfides and selenides formed by phosphorus. Although the structures of the sulfides (Fig. 15.24) appear to be closely related to those of the oxides (Section 15.10), there are some notable differences, e.g. P4O6 and P4S6 are not isostructural. The bond distances within the cages of aU the sulfides 

There is P NMR spectroscopic evidence that P4S8 has been prepared by treating P4S9 with PPh3. 

Phosphorus sulfides ignite easily, and P4S3 is used in strike anywhere matches it is combined with KCIO3, and the compounds inflame when subjected to friction. (In safety matches, the head of the match contains KCIO3 and this reacts with red phosphorus which is combined with glass powder on the side of the match box see end-of-chapter problem 15.42). Whereas P4S3 does not react with water, other phosphoms sulfides are slowly hydrolysed (e.g. reaction 15.154). 

When P2S5 is heated under vacuum with CS2S and sulfur in a 1 2 7 molar ratio, CS4P2S10 is formed. This contains discrete [P2Sio] ions (15.78), the terminal P S bonds in which are shorter (201pm) than the two in the central chain (219 pm). 

P2S7 can be stabilized by using pyridine (py) to form an adduct (py)2P2Sy forms when P4S10 is heated with Sg in pyridine. The P NMR spectmm of (py)2p2S7 shows one signal ( 82.2 ppm). Suggest a stmcture for the product and draw a resonance stmcture in which all atoms obey the octet mle. 

20 Schematic representations of the molecular structures of phosphorus sulfides, and the structure (X-ray diffraction) of P4S3 [L.Y. Goh et al. (1995) Organometallics, vol. 14, p. 3886], Colour code S, yellow P, brown. 

We have already noted Section 14.10) that, although sometimes referred to as phosphorus pentoxide , phosphorus(V) 

P4S7 + PhjP P4S6 + Ph3P=S excess sulfur 

The only well-characterized binary sulfide of Sb is the naturally occurring Sb2S3 (stibnite), which has a doublechain structure in which each Sb(III) is pyramidally sited with respect to three S atoms. The sulfide can be made by direct combination of the elements. A metastable red form can be precipitated from aqueous solution, but reverts to the stable black form on heating. Like AS2S3, Sb2S3 dissolves in alkali metal sulfide solutions (see equation 15.152). Bismuth(III) sulfide, 61283, is isostructural with 86283, but in contrast to its As and 8b analogues, 61283 does not dissolve in alkali metal sulfide solutions. 

---