Salt-like hydrides

The compounds to the left of the line in the following table are salt-like and contain H ions  

The alkali-metal hydrides have the NaCl structure, though-the positions of the H atoms have been confirmed only in LiH (X-ray diffraction) and NaH (neutron diffraction) (Li-H, 2-04 A, Na-H, 2-44 A). The rutile structure of MgHj has been established by neutron diffraction of MgD2 Mg-H, 1-95 A, shortest H-H, 2-49 A. 

The alkaline-earth hydrides all have a PbCl2 type of structure in which the metal atoms are arranged approximately in hexagonal closest packing. Of the two sets of non-equivalent H atoms one occupies tetrahedral holes while the other H atoms have (3 + 2)-coordination  

Note the tight packing of the H atoms Hi has 8 H neighbours at 2-50-2-94 A, and Hu has 10 H neighbours at 2-65-3-21 A. The shortest metal-metal distances in 

By the action of LiAlH4 on the halide (or in the case of Be on the dialkyls) a number of other hydrides have been prepared, including the dihydrides of Be, Zn, Cd, and Hg, trihydrides of Ga, In, and Tl, and CuH. Some of these are extremely unstable, those of Cd and Hg decomposing at temperatures below 0°C. Pure crystalline BeH2 has not been prepared, but a crystalline amine complex has been made for which the bridged structure (a) has been suggested Be—H bridges have 

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These give ionic or salt-like hydrides, for example 2Na + H2 2NaH... 

The interstitial hydrides of transition metals differ from the salt-like hydrides of the alkali and alkaline-earth metals MH and MH2, as can be seen from their densities. While the latter have higher densities than the metals, the transition metal hydrides have expanded metal lattices. Furthermore, the transition metal hydrides exhibit metallic luster and are semiconducting. Alkali metal hydrides have NaCl structure MgH2 has rutile structure. 

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. 

Most of the Group IA and IIA metals react with hydrogen to form metal hydrides. For all of the metals in these two groups except Be and Mg, the hydrides are considered to be ionic or salt-like hydrides containing H ions (see Chapter 6). The hydrides of beryllium and magnesium have considerable covalent character. The molten ionic compounds conduct electricity, as do molten mixtures of the hydrides in alkali halides, and during electrolysis of the hydrides, hydrogen is liberated at the anode as a result of the oxidation of H ... 

Table 2-7 The Salt-like Hydrides and Some of Their Properties...

The majority of the structures of compounds containing hydrogen are so distinctive in their properties that they are best discussed as a class apart (see chapter 12). These remarks, however, do not apply to the salt-like hydrides or to the hydroxides of the more electropositive metals, many of which form typically ionic structures resembling the corresponding halides in their properties. 

When we compare Aj77 for reaction 9.4 with those for the formations of F and CH from F2 and CI2 (—249 and —228kJmoP, respectively), we understand why, since H is about the same size as F, ionic hydrides are relatively unstable species with respect to dissociation into their constituent elements. Salt-like hydrides of metals in high oxidation states are most unlikely to exist. (More about binary hydrides in Section 9.7.)... 

Serendipitous use of measurements of hydrogen retained by metal powders formed by hydrogen reduction of salts has provided a likely explanation for these trends except for iron (no datum) and palladium (exothermic absorption), the trends were the opposite of those above. The amounts occluded are however greater by large factors than those due to simple dissolution, and the metals in question (unlike those in Groups 4 and 5) do not form salt-like hydrides. It was therefore... 

While the connection between the structure of the hydrides and their pyrophoric behavior seems to be obscure, it should be pointed out that the compounds of hydrogen fall into three distinctive groups the above-described phosphines, silanes, and boranes, which have covalent bonds the salt-like hydrides of the alkali and alkaline earth metals and the interstitial, nonstoicbiometric or berthollide —type hydrides of the transition metals—e.g. the rare earths, titanium, and zirconium. Pyrophoric compounds are found in all three groups. 

Electropositive elements form salt-like hydrides containing the hydride ion (H ), which are very reactive, nonmetals and transition metals form covalent hydrides such as methane (CH4), ammonia (NH3), and water (H2O). 

When, as in the case of LiH, they can be melted without decomposition, salt-like hydrides conduct electricity. On electrolysis, the melts yield hydrogen at the positive electrode. They all react with water, giving hydrogen and an alkaline solution ... 

Aluminium and beryllium hydrides react with water in the same way as the salt-like hydrides, to give hydrogen and metal hydroxides. However, the hydroxides are insoluble, so alkaline solutions are not produced for example... 

I and Group II except Be), macromolecular (Be and Al) or molecular (boron and Groups IV-VII/14-17.) Salt-like hydrides, which have the characteristics of ionic compounds, and macromolecular hydrides, react with water giving em aqueous or solid hydroxide. 

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