Metal phosphides phosphorus-rich

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... 

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. 

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) ... 

The numerous applications of phosphines include (1) synthetic reagents, (2) ligands in metallo-phosphorus compounds, (3) catalysts, (4) metal deposition agents, (5) electron-rich compounds. Metallophosphines (metal phosphides) types MPR2 and M2PR (M=Li, Na, K) are especially useful in synthesis (Chapter 8.8) (Table 6.8). 

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. 

In general, the sensitivity towards hydrolysis of metal phosphides depends on the metal content. Metal-rich compounds undergo relatively straightforward hydrolysis while the behavior of those with higher phosphorus content resembles more that of the elementary phosphorus, specially with respect to their inertness toward water. As observed in Table 4.16, many polynuclear metal phosphide undergo hydrolysis giving rise to corresponding phosphor hydrides or phosphanes. 

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. 

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... 

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... 

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