Ionic or Saline Hydrides

Hydrogen reacts direcdy with a number of metallic elements to form hydrides (qv). The ionic or saline hydrides ate formed from the reaction of hydrogen with the alkali metals and with some of the alkaline-eartb metals. The saline hydrides ate salt-like in character and contain the hydride, ie,, ion. Saline hydrides form when pure metals and H2 react at elevated temperatures (300—700°C). Examples of these reactions ate... 

Ionic Hydrides. The ionic or saline hydrides contain metal cations and negatively charged hydrogen ions. They crystallize in the cubic lattice similar to the corresponding metal halide, and when pure, are white sulid.s. When dissolved in molten sales or hydroxides and electrolyzed, hydrogen gas is liberated al the anode. Their densities arc greater than those of the parent metal, and their rormalion is exothermic. All are strong bases. 

Elements with very low electronegativeties of approximately 1.0 see Electronegativity), form compounds in which the hydrogen appears as an anion, H. These compounds have many of the properties associated with ionic substances and are sometimes called ionic or saline hydrides. Ionic hydrides, formed with alkali (M+H ) and alkaline earth (M +H2 ) metals, are colorless crystalline solids that either melt or decompose at temperatures above 600 °C. All of the hydrides can be formed by direct combination of the elements at elevated temperatures ... 

Unlike the halides that occur over a wide range of the periodic table, the ionic or saline hydrides are restricted to Group lA and 2A elements. To demonstrate quantitatively why this is the case, use the values of the bond energies and the electron affinities of hydrogen and fluorine found in Table 10.3 to calculate the energy of the following process in which X = H and F ... 

The foregoing remarks do not hold, of course, for the dihydrides of the triva-lent lanthanides. They exhibit metallic conduction (10), as would be expected, since their conduction band is only somewhat depleted. One would expect them to display a tendency to order at low temperatures, but it seems not unreasonable to expect that this tendency would be weaker than the corresponding element, in view of the decreased electron concentration, and the ordering would hence occur at lower temperatures. This was in fact observed for HoH2, which exhibits (6) a Neel point at 8° K., as coippared to 135° K. for Ho. It is also observed in the present work for the terbium dihydrides, whose Neel points are 40° to 50° K., whereas that for the element is 241° K. These properties are compatible with the notion that hydrogen in all the lanthanide hydrides is anionic. On this basis the dihydrides appear as an intermediate form between the truly metallic elements on the one hand and the truly ionic or saline trihydrides on the other. 

The nature of a binary hydride is related to the characteristics of the element bonded to hydrogen (Fig. 14.8). Strongly electropositive metallic elements form ionic compounds with hydrogen in which the latter is present as a hydride ion, H. These ionic compounds are called saline hydrides (or saltlike hydrides). They are formed by all members of the s block, with the exception of beryllium, and are made by heating the metal in hydrogen ... 

The saline hydrides do not show appreciable deviations from stoichiometry at room temperature. Lithium hydride, which has been studied more extensively, exhibits a slight deficiency in hydrogen. Since the saline hydrides are predominantly ionic, variation from the stoichiometric composition must involve a valence change, which can manifest itself either as F-centers (42) or colloidal (8, 42) lithium. 

Thus, owing to the endothermic character of the H" ion, only the most electropositive metals—the alkalis and the alkaline earths—form saline or saltlike hydrides, such as NaH and CaH2. The ionic nature of the compounds is shown by their high conductivities just below or at the melting point and by the fact that on electrolysis of solutions in molten alkali halides hydrogen is liberated at the anode. 

The hydrides formed in reaction (a) may be classified as (1) saline or ionic hydrides, (2) metallic hydrides and (3) covalent hydrides. The saline hydrides include the hydrides of the alkali and alkaline-earth metals, except BeHj, which is covalent. Transition metals form binary compounds with hydrogen that are classified as metallic hydrides including rare-earth and actinide hydrides. Intermetallic compound hydrides, such as TiFeHj and LaNijH, may be thought of as pseudobinary metallic hydrides. 

Valence and oxidation state are directly related to the valence-shell electron configuration of a group. Binary hydrides are classified as saline, metallic, or molecular. Oxides of metals tend to be ionic and to form basic solutions in water. Oxides of nonmetals are molecular and many are the anhydrides of acids. 

This chapter commences with a review of a limited number of ternary hydride systems that have two common features. First, at least one of the two metal constituents is an alkali or alkaline earth element which independently forms a binary hydride with a metal hydrogen bond that is characterized as saline or ionic. The second metal, for the most part, is near the end of the d-electron series and with the exception of palladium, is not known to form binary hydrides that are stable at room temperature. This review stems from our own more specific interest in preparing and characterizing ternary hydrides where one of the metals is europium or ytterbium and the other is a rarer platinum metal. The similarity between the crystal chemistry of these di-valent rare earths and Ca2+ and Sr2+ is well known so that in our systems, europium and ytterbium in their di-valent oxidation states are viewed as pseudoalkaline earth elements. 

The ionic hydrides are white solids with high melting points, and all of the alkali metal hydrides have the sodium chloride crystal structure. Because they resemble the salts of the alkali and alkaline earth metals, the ionic hydrides are often referred to as saline or salt-like hydrides. The properties of the alkali metal hydrides are shown in Table 6.3, and those of the alkaline earth hydrides are shown in Table 6.4. 

The alkali-metal hydrides are colorless solids that crystallize in the NaCl cubic system. They are saline or ionic hydrides containing an M" " cation (M = Li, Na, K, Rb, Cs) and the H hydride anion, the dimensions of which are comparable to those of the fluoride ion. ... 

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