Phosphorus-Rich Phosphides

Phosphorus-rich phosphides of most metals other than alkali or alkaline earths (above) contain polymerised P atoms, but cannot be completely satisfactorily represented by either ionic or covalent formulae. In their structures each P atom is usually linked to at least one other P atom and up to three metal atoms, in at least an approximate tetrahedral configuration, at distances expected for covalent bonds. 

On heating, these compounds lose phosphorus and usually revert to a monophosphide or a metal-rich phosphide. Semiconductor properties are frequently found amongst these compounds. They are 

FIGURE 8.12 Structures of (a) PtP2 (unit cell) and (b) RI1P3 (part unit cell). Completed unit cell consists of 8 adjacent cube sub units, 6 of which are filled by planar P4 rings as indicated, with remaining sub units being empty. 

If alternative ionic formulations are used, IrPj (8.25b) contains rings and SnPj (8.25a) contains some Pg rings. 

The ternary phosphide Cu4SnPio, prepared by direct synthesis from the elements, contains the highly symmetrical adamantane-type Pjq cage unit (8.25b). Each P atom is tetrahedrally coordinated to metal atoms and other P atoms at distances corresponding to covalent linkages. If the structure is regarded as ionic, the anion corresponds to Pjo . 

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Phosphorus production technology, 79 5 Phosphorus production plants, 79 17 Phosphorus removal, as advanced wastewater treatment, 25 907 Phosphorus-rich phosphides, 79 59 Phosphorus selenides, 22 87 Phosphorus sesquisulfide, 79 47 Phosphorus-silver, UNS designation,... 

Finally, it is possible to obtain phosphorus-rich phosphides by the reaction of P4 with Li3P7 ... 

Phosphides resemble in many ways the metal borides (p. 145), carbides (p. 297), and nitrides (p. 417), and there are the same difficulties in classification and description of bonding. Perhaps the least-contentious procedure is to classify according to stoichiometry, i.e. (a) metal-rich phosphides (M/P > 1), (b) monophosphides (M/P =1), and (c) phosphorus-rich phosphides (M/P < 1) ... 

Among binary transition-metal pnictides, only the first-row transition-metal phosphides have been analysed by XPS extensively, whereas arsenides and antimonides have been barely studied [51-61]. Table 2 reveals some general trends in the P 2p3/2 BEs for various first-row transition-metal monophosphides, as well as some metaland phosphorus-rich members forming for a given transition metal. Deviations of as much as a few tenths of an electron volt are seen in the BEs for some compounds measured multiple times by different investigators (e.g., MnP), but these... 

It is known from the preceding research that phosphorus-rich silylphosphanes or their related lithium phosphides undergo, with LiBu in ether, reactions in which a structural transformation occurs, as shown in Scheme 16 20). The reactions of the partially silylated tri- and cyclotetraphosphanes were explored in order to come closer to understanding the above reactions. It can be taken for granted that the P—C bond is not affected in such reactions. 

Fhst, the number of valence functions (or frontier orbitals) and valence elechons (frontier orbital occupancy) determines the tendency toward cluster bonding. It is instructive to recall that the structural motif in elemental boron is the icosahedron with six-connected boron atoms see Borides Solid-state Chemistry), it is the tetrahedral carbon atom in the diamond form of elemental carbon with four-coimected carbon atoms and it is three-connected phosphorus atoms in the sheets of elemental black phosphorus (see Phosphides Solid-state Chemistry). Boron has more valence orbitals than valence elechons, naturally leading to orbitally rich cluster formation for example, BH has three orbitals and two elechons and forms... 

We have emphasized here the existence of P-P bonds in the phosphorus-rich compounds. The existence of metal-metal bonds in the lower phosphides is also of interest, since this has a direct bearing on their magnetic properties. 

Eor the group III phosphides InP and GaP, as well as their ternary alloy InGaP, two main surface symmetries have been found for samples prepared under UHV conditions a (2 x 1) surface structure under phosphorus-rich surface conditions and a (2x4) reconstraction in the case of group-111-rich conditions. We will discuss these two surface reconstructions for the case of InP(OOl) and compare these results afterward to those of GaP(OOl) and InGaP(OOl). 

Further products of the hydrolysis of calcium phosphide are P2H4, P3H5 and other hydrides, all of which contain P—P bonds. These are very sensitive to air and also are very unstable, tending to decompose to PH3 and phosphorus-rich polymers. The most stable is P7H3, which has the cage structure shown in Figure 11.16. Its stability is due to the wide separation of the hydrogen atoms, so that PH3 is not lost readily. 

Up to now, metal phosphides have mainly been obtained either by solid-state synthetic techniques, under extreme conditions with long reaction times, or via the molecular phase to enable their use in the preparation of new materials. However, the renaissance and subsequent development of the chemistry of catenated oligophosphanide anions and their metal complexes may open up novel routes for the preparation of metal phosphides by thermal treatment of phosphorus-rich metal complexes. 

Table 4.1). In the mass spectmm of 38, the organyl-free fragment [PtPJ" was observed, and hence this complex may be a suitable molecular precursor for the synthesis of binary phosphorus-rich platinum phosphides. 

The ability of phosphorus to exist as isolated anions or larger anionic polyphosphide networks with P-P bonds enables possible formation of transition metal phosphides [14], which are an important class of binary metal/non-metal compounds with a wide variety of structures, compositions and properties. For example, phosphorus-to-metal ratios in these compounds have a wide range, from metal-rich (MPj where v < 1) to monophosphides (MP) and phosphorus-rich polyphosphides (MPj where x > 1) [93]. 

Reports on phosphorus-rich metal phosphides are rare compared to their metal-rich analogues. This may be due to the problems associated with the preparation of these materials, which are caused by the lower thermal stability compared to metal-rich phosphides. The stability problems limit the possibility to make metal-rich phosphides by simply heating a mixture of the elements at elevated temperature [101, 127-129]. 

Some of the reported phosphorus-rich metal phosphides have been synthesised from the elements (powdered metal and red phosphorus) at elevated temperatures (700-1,200 °C), or at moderate temperatures for extended periods of time in tin fluxes (up to 550 °C, more than 10 days) [110, 127, 130-132]. These species can also be prepared by heating the elements in the presence of a chemical transport agent such as CI2 or I2 in sealed ampoules (between 600 and 800 °C)... 

Alternative routes that seem feasible are decomposition of the corresponding phosphorus-rich metal oligophosphanide complexes under mild conditions, and reactions of metal salts and neutral phosphorus-rich phosphanes under solvothermal conditions. These studies may lead to the preparation of novel phosphorus-rich metal phosphides whose potential as anode materials for lithium-ion batteries [134-136, 141-146] and thermoelectric materials may be studied [139, 140]. One of the principal processes for the preparations of metal phosphides starting from phosphorus-rich metal ohgophosphanide complexes is removal of the R group, which should be possible by thermal decomposition of the compounds, to give organyl-free species such as M Py. These species have already been observed in the mass spectra of several of these complexes. 

Barry BM, Gillan EG (2008) Low-temperature solvothermal synthesis of phosphorus-rich transition-metal phosphides. Chem Mater 20 2618-2620... 

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