Metal Phosphide Borides

Compounds such as M5PB2 (M = Fe, Mn, Co) are isostructural derivatives of CrjB3. 

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The metal-rich transition metal phosphides (<60% P) are dark coloured and insoluble in water, they have high chemical and thermal stability, they are dense, hard and brittle and have high thermal and electrical conductivities. These properties they have in common with the transition metal borides and silicides (and in some cases carbides and nitrides) to which they are often structurally related. With few exceptions, the transition metal phosphides, borides and silicides are not attacked by dilute acids and bases and may remain unaffected by hot concentrated mineral acids. 

Among the remarkably diversified variety of binary and ternary meta boride structures, the triangular prismatic coordination MgB has the dominating role, whereas among metal phosphides, sillcides and carbides this is merely true for a limited series of structure types. 

Phosphides, in general, tend to have lower melting points and lower stabilities, and to be less hard than the corresponding silicides, borides and other metallides. Metal phosphides exhibit a whole range of colours from colourless to black. 

In each chapter, the following sequence in order of classes of compounds has been adopted metal-like borides, carbides, nitrides, silicides, phosphides, sulfides, and nonmetallic compounds. 

Binary transition metal boride Binary transition metal phosphide Data for hypothetical compound assuming ordering of type a) only ... 

Phosphides resemble in many ways the metal borides (p. 145), earbides (p. 297), and nitrides (p. 417), and there are the same diffieulties in elassifieation and deseription of bonding. Perhaps the least-eontentious proeedure is to elassify aeeording to stoiehiometry, i.e. (a) metal-rieh phosphides (M/P >1), (b) monophosphides (M/P =1), and (e) phosphorus-rieh phosphides (M/P < 1) ... 

Such reactions are discussed at appropriate points throughout the book as each individual compound is being considered. A particularly important set of reactions in this category is the synthesis of element hydrides by hydrolysis of certain sulfides (to give H2S), nitrides (to give NH3), phosphides (PH3), carbides (C Hm), borides (B Hm), etc. Useful reviews are available on hydrometallurgy (the recovery of metals by use of aqueous solutions at relatively low temperatures), hydrothermal syntheses and the use of supercritical water as a reaction medium for chemistry. 

Low-temperature solvents are not readily available for many refractory compounds and semiconductors of interest. Molten salt electrolysis is utilized in many instances, as for the synthesis and deposition of elemental materials such as Al, Si, and also a wide variety of binary and ternary compounds such as borides, carbides, silicides, phosphides, arsenides, and sulfides, and the semiconductors SiC, GaAs, and GaP and InP [16], A few available reports regarding the metal chalcogenides examined in this chapter will be addressed in the respective sections. Let us note here that halide fluxes provide a good reaction medium for the crystal growth of refractory compounds. A wide spectrum of alkali and alkaline earth halides provides... 

II-VI and III-V compounds, borides, carbides, nitrides and silicides of transition metals, as well as sulphides, phosphides, aluminides, etc. A1203, AIN, B203, BN, SiC, Si3N4, U02, Y203, Zr02, etc. 

Electronic conductance is characteristic for the so called conductors of the first class, i. e. for metals (both in solid and fused state) and some metal oxydes, carbides, sulphides, phosphides and borides and it can be explained by assuming the existence of free electrons which act in solid matter as anions. Under the influence of the external electric field these easily movable electrons start an ordered motion while the atoms deprived of their electrons, which are in fact cations, take practically no part in the current conduction and, apart from their vibration within the mean equilibrium positions, remain practically immobile. The passage of the current does not manifest itself by a chemical change of the... 

Boron phosphide occurs in two forms, one of which, cubic BP, has a diamondlike structure analogous to cubic boron nitride (see above). The other variety, Bi2PL8, has a partially disordered crystal structure that contains icosahedral Bi2 units, as found in many metal borides (Section 5-3). Cubic BP is extremely inert, resisting attack by boiling concentrated acids or bases, is not oxidized in air below... 

The second example comes from solid-state chemistry, and the coimection with cluster chemistry is less obvious. Despite this, it is an excellent example of how E/M variation can lead to systematic variation in structure and, consequently, to properties. Although the example is taken from solid-state metal borides, the silicides see Silicon Inorganic Chemistry) and phosphides (see Phosphides Solid-state Chemistry) could have been used. 

Hydrogen reacts with metal borides, carbides, silicides, nitrides, phosphides, oxides, sulfides, and halides to form a solid solution of hydrogen in the compound with... 

X-ray photoelectron (XPS) studies of nickel boride, nickel phosphide, Raney nickel and Urushibara nickel showed that the electron density on the nickel was a function of the other metal present in these catalysts. 28J29 Boron, aluminum (Raney nickel) and zinc (Urushibara nickel) all increased the electron density on the nickel while phosphorous was an electron acceptor. Comparing the electron densities on the nickel in these catalysts with that on a nickel black prepared by the thermal decomposition of nickel formate (D-Ni) gave the series Ni-B > Ni-Al > Ni-Zn > D-Ni > Ni-P. 

Inorganic non-oxide materials, such as III-V and II-VI group semiconductors, carbides, nitrides, phosphides, and borides, are traditionally prepared by solid state or gas-phase reactions. They have also been prepared via the pyrolysis of organo-metallic precursors but the products may be amorphous or poorly crystalline and a crystallization treatment at higher temperature is necessary. This treatment, however, may result in the crystalline size being beyond the nanometer scale. Exploration of milder techniques for preparing non-oxide nanomaterials with controlled shapes and sizes is very important for material science. 

In the fifteen years since publication of the first edition of Comprehensive Coordination Chemistry (CCC, 1987), group 5 chemistry has been part of the intensive development of ceramic, optical, and magnetic materials based upon metal borides, nitrides, phosphides, oxides, and sulfides. A major impetus came from the discovery of the high-temperature superconducting oxides. In addition, the search for new routes to these materials via sol-gel or chemical vapor deposition techniques has spurred growth in metal amido, oxo, alkoxo, thio, and carboxylato chemistry. 

The bulk metal is oxidized by air or steam only at high temperatures, but Raney nickel (see Section 21.2) is pyrophoric. Nickel reacts with F2 to give a coherent coating of NiF2 which prevents further attack hence the use of nickel and its alloy Monel metal in apparatus for handling F2 or xenon fluorides. With CI2, Bt2 and I2, Ni(II) halides are formed. At elevated temperatures, Ni reacts with P, S and B and a range of different phosphide (see Section 14.6), sulfide and boride (see Section 12.10) phases are known. 

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