Hydrides binary

The most common moderators are substances of low atomic weight such as heavy water (deuterium oxide) or graphite. Hydrides (binary compounds corrtaining hydrogen), hydrocarborrs, and beryllium and beryllium oxide have also been used as moderators in certain specialized kinds of reactors. 

Another option is using hydrides, binary compounds of hydrogen with one other substance. Above all metal hydrides offer themselves as storage medium. Their advantages are ... 

The lattice gas approach is valid within certain limits for typical metallic hydrides, binaries as well as ternaries. Deviation from this idealized picture indicates that metallic hydrides are not pure host-guest systems, but real chemical compounds. An important difference between the model of hydrogen as a lattice gas, liquid, or solid and real metal hydrides lies in the nature of the phase transitions. Whereas the crystallization of a material is a first-order transition according to Landau s theory, an order-disorder transition in a hydride can be of first or second order. The structural relationships between ordered and disordered phases of metal hydrides have been proven in many cases by crystallographic group-subgroup relationships, which suggests the possibility of second-order (continuous) phase transitions. However, in many cases hints for a transition of first order were found due to a surface contamination of the sample that kinetically hinders the transition to proceed. 

Figure 11.24A shows a plot of boiling point versus molecular weight for the hydrides (binary compounds with hydrogen) of the Group VIA elements. If London... 

Binary Compounds of Hydrogen. Binary compounds of hydrogen with the more electropositive elements are designated hydrides (NaH, sodium hydride). 

Hydrides are compounds that contain hydrogen (qv) in a reduced or electron-rich state. Hydrides may be either simple binary compounds or complex ones. In the former, the negative hydrogen is bonded ionicaHy or covalendy to a metal, or is present as a soHd solution in the metal lattice. In the latter, which comprise a large group of chemical compounds, complex hydridic anions such as BH, A1H, and derivatives of these, exist. 

The metallic monohaHdes zirconium chloride [14989-34-5] ZrCl, and zirconium bromide [31483-18-8] ZrBr, reversibly absorb hydrogen up to a limiting composition of ZrXH (131). These hydrides are less stable than the binary hydride ZrH2, and begin to disproportionate above 400°C to ZrH2 and ZrX in a hydrogen atmosphere (see also Hydrides). 

Arsenic Hydrides. Although there are occasionally reports of other arsenic hydrides, eg, AS2H4, AS2H2 (or AsH), and AS4H2, the only weU-characterized binary compound of arsenic and hydrogen is arsine. 

Thus the hydride is a very efficient carrier of hydrogen. Upon heating, calcium reacts with boron, sulfur, carbon, and phosphoms to form the corresponding binary compounds and with carbon dioxide to form calcium carbide [73-20-7J, CaC2, and calcium oxide [1305-78-8] CaO. 

Various trends have long been noted in the aeid strengths of many binary hydrides and oxoaeids. Values for some simple hydrides are given in Table 3.4 from whieh it is elear that aeid strength inereases with atomie number both in any one horizontal period and in any... 

Fig. 3.11. Numerous examples are also known in which hydrogen acts as a bridge between metallic elements in binary and more complex hydrides, and some of these will be mentioned in the following section which considers the general question of the hydrides of the elements.

Hydrogen combines with many elements to form binary hydrides MH (or M H ). All the main-group elements except the noble gases and perhaps indium and thallium form hydrides, as do all the lanthanoids and actinoids that have been studied. Hydrides are also formed by the more electropositive transition elements, notably Sc, Y, La, Ac Ti, Zr, Hf and to a lesser... 

This section deals with the binary compounds that nitrogen forms with metals, and then describes the extensive chemistry of the hydrides, halides, pseudohalides, oxides and oxoacids of the element. The chemistry of P-N compounds is deferred until Chapter 12 (p. 531) and S-N... 

The binary borides (p. 145), carbides (p. 299), and nitrides (p. 418) have already been discussed. Suffice it to note here that the chromium atom is too small to allow the ready insertion of carbon into its lattice, and its carbide is consequently more reactive than those of its predecessors. As for the hydrides, only CrH is known which is consistent with the general trend in this part of the periodic table that hydrides become less stable across the d block and down each group. 

Hydrido complexes are well-known bui simple binary hydrides are noi. which is iti keeping wiih ihe posinon nf ihese melals in Ihe hydrogen gap" portion of ihe periodic lable ip, 67. ... 

The borides (p. 145), carbides (pp. 297, 1074), and nitrides (p. 417) have been discussed previously. Binary hydrides are not formed but prolonged heating of powdered Mg and Fe under a high pressure of H2 yields MgFeH6 containing the octahedral hydrido anion, [FeH6] which satisfies the 18-electron rule. 

Because they possess an odd number of valence electrons the elements of this group can only satisfy the 18-electron rule in their carbonyls if M-M bonds are present. In accord with this, mononuclear carbonyls are not formed. Instead [M2(CO)s], [M4(CO)i2] and [M6(CO)i6] are the principal binary carbonyls of these elements. But reduction of [Co2(CO)g] with, for instance, sodium amalgam in benzene yields the monomeric and tetrahedral, 18-electron ion, [Co(CO)4] , acidification of which gives the pale yellow hydride, [HCo(CO)4]. Reductions employing Na metal in liquid NH3 yield the super-reduced [M(CO)3] (M = Co, Rh, Ir) containing these elements in their lowest formal oxidation state. 

Such binary borides (p. 145), carbides (p. 297) and nitrides (p. 417) as are formed have been referred to already. The ability of the metals to absorb molecular hydrogen has also been alluded to above. While the existence of definite hydrides of nickel and platinum is in doubt the... 

Switendick was the first to apply modem electronic band theory to metal hydrides [5]. He compared the measured density of electronic states with theoretical results derived from energy band calculations in binary and pseudo-binary systems. Recently, the band structures of intermetallic hydrides including LaNi5Ht and FeTiH v have been summarized in a review article by Gupta and Schlapbach [6], All exhibit certain common features upon the absorption of hydrogen and formation of a distinct hydride phase. They are ... 

In order for an intermetallic compound to react directly and reversibly with hydrogen to form a distinct hydride phase, it is necessary that at least one of the metal components be capable of reacting directly and reversibly with hydrogen to form a stable binary hydride. 

If the metal atoms are not mobile (as is the case in low—temperature reactions) only hydride phases can result in which the metal lattice is structurally very similar to the starting intermetallic compound because the metal atoms are essentially frozen in place. In effect the system may be considered to be pseudo-binary as the metal atoms behave as a single component. 

G. G. Libowitz, The Solid State Chemistry of Binary Metal Hydrides, W. A. Benjamin, New York, 1965. 

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