Hydrides saline

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... 

The saline hydrides ate very reactive and ate strong reducing agents. AH saline hydrides decompose in water, often violently, to form hydrogen ... 

Catalysts can be beneficial in the preparation of some saline hydrides (116). 

The compounds formed by the reaction of hydrogen with the alkali and alkaline earth metals contain H- ions for example, sodium hydride consists of Na+ and H- ions. These white crystalline solids are often referred to as saline hydrides because of their physical resemblance to NaCL Chemically, they behave quite differently from sodium chloride for example, they react with water to produce hydrogen gas. Typical reactions are... 

In this way, saline hydrides can serve as compact, portable sources of hydrogen gas for inflating life rafts and balloons. 

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 are white, high-melting-point solids with crystal structures that resemble those of the corresponding halides. The alkali metal hydrides, for instance, have the rock-salt structure (Fig. 5.39). 

Because this reaction produces hydrogen, saline hydrides are potentially useful as transportable sources of hydrogen fuel. 

The saline hydrides all react rapidly with water. They also react similarly with liquid ammonia, (a) Write a balanced equation for the reaction of CaH, with liquid ammonia. 

Draw simple molecular orbital energy-level diagrams to indicate how the bonding in the saline hydrides, such as NaH or KH, differs from that between hydrogen and a light p-block element such as carbon or nitrogen. 

C. E. Messer, Saline Hydrides in Preparative Inorganic Reactions, Interscience Publishers Inc., New York 1964. 

Salicylic alcohol glucosides, natural, 22 7 Salicyloyl chloride, 22 3 Salicylsalicylic acid, 22 16-17 physical properties of, 22 15t Saligenin, 22 23, 24 Salina salt, 5 788 Saline hydrides, 13 771 Saline solutions... 

NoH, LiAIHJ. For instance, they can act as reducing agents toward many organic compounds and are capable of hydrogenating aikynes and alkenes. Thus their H ligands are intermediate between the strictly hydridic hydrogens in the saline hydrides and the protonic hydrogens in compounds with nonmetals (e.g.. HCI, NH3). 

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. 

The properties of the nonstoichiometric hydrides are discussed in groups the saline hydrides, the Group IVA hydrides, the rare earth hydrides, the actinide hydrides, the Group VA hydrides, and palladium hydride. 

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. 

The hydrides show the chemical and physical properties one would anticipate from the position of the metal in the periodic table and from the foregoing. The strongly electropositive metals of the first two groups give saline hydrides which physically resemble the corresponding fluorides. The transition metals form metallic or semimetallic hydrides, presumably because of their ability to use d or d-hybrid orbitals which are delocalized into a conduction band. Apart from their metallic character, however, there are no unexpected differences in proper-... 

Acquisition of an Electron. The hydrogen atom can acquire an electron, attaining the Is2 structure of He, to form the hydride ion H . This ion exists as such essentially only in the saline hydrides formed by electropositive metals (Section 2-13). 

The saline hydrides are prepared by direct interaction at 300 to 700°C. The rates are in the order Li > Cs > K > Na, largely because of the activation energy term in the Arrhenius equation. Extremely active hydrides of Li, Na, and K can be made by interaction of hydrogen with BuLi + TMEDA in hexane (LiH) or of BuLi + M(OBu ) + TMEDA in hexane (NaH and KH). 

The saline hydrides are crystalline solids, white when pure but usually gray owing to traces of metal. They can be dissolved in molten alkali halides and on electrolysis of such a solution, for example, CaH2 in LiCl + KC1 at 360°C, hydrogen is released at the anode. They react instantly and completely with even the weakest acids, such as water, according to the reaction... 

Zintl compounds to be mentioned in other chapters are compounds or phases that have anions such as Sng-, Sn2Bi2 or Pb1. The alkali metal salts can often be isolated crystalline by complexation of the cation with crown ethers or cryptands. Recent examples are Na jSn16 and Li2Ba4Si6, the latter having a SiJ0- ring.17 Saline hydrides were discussed in Chapter 2. 

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 ... 

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. 

The crystal lattices of the saline hydrides consist of hydrogen anions and metal cations, but not exclusively e.g., in LiH, calculations and diffraction experiments suggest that electron transfer from Li to H is 0.8-1 e, implying a strong ionic bond with covalent character. Magnesium hydride occupies a special position. Although classified here as a saline hydride, its properties are intermediate between the ionic hydrides and covalent BeHj-... 

Hydrides are broadly of three types, saline, covalent and metallic. Saline hydrides are formed by the alkali metals (Gp. lA), the alkaline earths (Gp. IIA) and the lanthanides they have ionic lattices, high melting points and, when fused, are electrolytes. Elements of the B Groups from IIIB to VIIB have covalent hydrides, most of them gaseous at room temperature. The metallic hydrides characteristic of some of the transition elements are in effect alloys and usually lack the stoichiometric composition of normal chemical compounds. 

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