Elements binary hydrides

Hydrogen forms binary compounds, called hydrides, with most of the other elements. Binary hydrides are usually grouped into three broad categories covalent, ionic, and metallic. Covalent hydrides are those formed between... 

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... 

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 ... 

Fio. 6. Proton-shielding parameters for the binary hydrides of the elements plotted against the atomic number t>f the central atom. Explanation of symbols ... 

Figure 5.20 Table of the binary hydrides and the Allred-Rochow electronegativity [26], Most elements react with hydrogen to form ionic, covalent or metallic binary hydrides.

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. 

Alternative names for cations obtained formally by the addition ofhydrons to mono nuclear binary hydrides. Names for these simple cations can be derived as described above. Alternatively, they may be named by adding the ending -onium to a stem of the element name. 

The periodic trends in main-group elements become apparent when we compare the binary compounds they form with one specific element. All the main-group elements, with the exception of the noble gases and, possibly, indium and thallium, form binary compounds with hydrogen, so these hydrides can be examined to look for periodic trends. We meet the binary hydrides several times in this chapter and the next, so, at this stage, we confine the discussion to a brief survey and see how their properties reveal periodic behavior. 

The binary hydrides are compounds that contain hydrogen and just one other element. Formulas and melting points of the simplest hydrides of the main-group elements are listed in Figure 14.2. Binary hydrides can be classified as ionic, covalent, or metallic. 

The following pictures represent binary hydrides AHX, where A = K, Ti, C, or F. Ivory spheres represent H atoms or ions, and burgundy spheres represent atoms or ions of the element A. 

The following models represent the structures of binary hydrides of second-row elements ... 

Apart from the binary hydrides of Groups 16 and 17, Lowry/ Brpnsted acids in aqueous solution are nearly all oxoacids, i.e. substances containing O-H bonds which ionise in aqueous solution to give oxo-anions and H+(aq) (or H30+). Most oxoacids are molecular hydroxides E(OH) , such as B(OH)3, Ge(OH)4 and Te(OH)6, or oxohydroxides EOm(OH) . In addition, we have more complex species containing E-E bonds or E-O-E bridges. In EOm(OH) - for example, N02(0H), PO(OH)3, S02(0H)2,103(0H) - the m O atoms are held to E by bonds having at least some double bond character, via p -p or d -p overlap. Oxohydroxides may be seen as being derived from hydroxides by elimination of H20, and are favoured by elements E whose atoms form double bonds to O atoms. 

The most obvious route to a binary compound ExE y is the direct interaction between the elemental substances E and E. This is often precluded by thermodynamic considerations however, a great many compounds that are unstable with respect to the elemental substances are kinetically stable and can be prepared by other means. Even if the reaction between the elemental substances is possible, both from thermodynamic and kinetic viewpoints, an indirect synthesis may be preferable for practical reasons. We shall illustrate these principles by looking at some examples of synthetic routes to binary hydrides and halides. 

The direct formation of a binary hydride EH from the elemental substances is usually favoured thermodynamically if E is much lower, or much higher, in electronegativity than H. Thus, for example, the free energies of formation AG of LiH and HC1 are respectively —68 and -95 kJ mol-1. In the case of HC1, it can be predicted from the relevant bond energies - rationalised in terms of Pauling s derivation of electronegativities - that the formation of HCl(g) from the elemental substances should be exothermic. Further, entropy considerations will favour the randomisation of atoms in molecules. For the reaction ... 

Binary compounds with less electronegative elements include hydrides, nitrides, sulphides and phosphides. They are decomposed by water and can provide convenient routes for the preparation of non-metal hydrides. The anions may be polyatomic or polymerised, as with CaC2, which contains C22- and reacts with water to give acetylene (ethyne) C2H2. 

Generally chemists recognize three broad types of binary hydrides (that is, compounds containing hydrogen and one additional element). These are molecular hydrides, saltlike hydrides, and interstitial hydrides. 

Needless to say, when the hydrides have a resemblance in the chemical bonding state, their locations are close to each other in the diagram. For example, any binary hydrides of transition elements appear in the higher AEh region than those of typical elements. This indicates that transition elements could... 

Recently, Grochala and Peters have shown that the thermal decomposition temperature of many binary hydrides could be correlated with the standard redox potential for the metal cation/metal redox couple and with the standard enthalpy of decomposition [18, 83[. They also showed that, for multinary hydrides, the decomposition temperature could be tuned by a careful choice of the stoichiometric ratio and of the Lewis acid/base character of the constituent elements. 

Hydrogen combines with many elements to form binary hydrides, MH . There are three general classes of hydrides ... 

Other binary hydrides can be formed with light elements, without improvement in performance. The structure of NiH is shown in Fig. 2.51. For heavy metals, only palladium has received some attention, due to rapid absorption and desorption at ambient temperature and pressure. However, only 0.6% hydrogen by mass is stored, which together with high metal cost makes this option unattractive. 

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