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 

WiBERC and E. Awbercer. Hydrides of the Elements ef Main Groups I-IV, Elsevier. Amsterdam. 1971. 785 pp. w. M. Mueller, J. P. Blackledge and G. G. Libowitz (eds.), Metal Hydrides, Academic Press, New York, 1968, 791 pp. 

TIu - Palladium-Hydrogen System, Academic Press, London, 1967, 178 pp. 

RbH SrH] VH, /rHi NhHa Me T hu Kh Pilll l As (CdHj (InH 1 huRi SnjHa ShHi TeHi 111 X 

The metals in Groups I and 2 of the periodic table react directly with hydrogen to form white, crystalline, stoichiometric hydrides of formula MX and MX2 respectively. The salt-like character of these compounds was recognized by G. N. Lewis in 1916 and he suggested that they contained the hydride ion H . Shortly thereafter 

Lewis in 1916 and he suggested that they contained the hydride ion H . Shortly thereafter 

The closest M-M approach in these compounds is often less than for the metal itself this should occasion no surprise since this is a common feature of many compounds in which there is substantial separation of charge. For example, the shortest Ca-Ca interatomic distance is 393 pm in calcium metal, 360 pm in CaH2, 380 pm in Cap2, and only 340 pm in CaO (why ). 

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The element before carbon in Period 2, boron, has one electron less than carbon, and forms many covalent compounds of type BX3 where X is a monovalent atom or group. In these, the boron uses three sp hybrid orbitals to form three trigonal planar bonds, like carbon in ethene, but the unhybridised 2p orbital is vacant, i.e. it contains no electrons. In the nitrogen atom (one more electron than carbon) one orbital must contain two electrons—the lone pair hence sp hybridisation will give four tetrahedral orbitals, one containing this lone pair. Oxygen similarly hybridised will have two orbitals occupied by lone pairs, and fluorine, three. Hence the hydrides of the elements from carbon to fluorine have the structures... 

Discuss the chemistry of the simple hydrides of the elements, indicating how they can be classified according to their structures. 

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.

E. WlBERO and E. Amberoer, Hydrides of the Elements of Main Groups I-IV, Chap. 7, pp. 462- 638, Elsevier, Amsterdam, 1971. A comprehensive review of compounds containing Si-H bonds over 700 references. 

FIGURE 14.7 The chemical formulas of the hydrides of the elements of the main groups display the valence characteristic of each group. 

The situation does not improve with mixtures with the hydrides of the elements. Thus, a detonation occurred during contact between water and chlorine due to an accidental spark. Phosphine, silane and diborane all combust spontaneously in chlorine (their behaviour is the same in oxygen). With hydrogenated nitrogenous compounds ammonia, hydrazine, hydroxylamine, ammonium salts (especially ammonium chloride), and also sulphamic acid (these last two in an acid medium) there is ignition or even detonation. 

Next, we illustrate these properties by discussing them together with some of the atomic properties for the diatomic hydrides of the elements of periods 2 and 3. 

In this section we discuss the bonding of the fluorides, chlorides, and hydrides of the elements of periods 3 and beyond with LLP coordination numbers up to four with particular emphasis on the elements of period 3. As might be expected these molecules show many similarities to the corresponding period 2 molecules, and the differences can be mainly attributed to the larger size and lower electronegativity of the atoms of a period 3 element compared to the corresponding period 2 element. 

This survey has shown that the homogeneous decomposition of hydrides of the elements becomes more complex when moving from right to left in the main... 

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 ... 

HYDRIDES OF THE ELEMENTS IN GROUP 4A ACTING AS PROTON ACCEPTORS AND PROTON DONORS VERY WEAK DIHYDROGEN BONDS... 

Wiberg, E. and Amberger, E. (1971) Hydrides of the Elements of Main Croups I—IV, Elsevier, Amsterdam. 

Acid strengths of the hydrides of the elements of a period increase from left to right across the period in the same order that electronegativity increases in the periodic table. A highly electronegative element repels an electron of hydrogen atom, and releases it as hydrogen (H" ) ion. Let us consider N, O and F. 

Table 1.1 Melting and boiling temperalures (°C) for hydrides of the elements of Groups 14 and 16...

Conductivity measurements in N/10 aqueous solution show the dissolved gas to be ionised to the extent of 50 per cent., whilst hydrogen selenide in Nj 10 solution is only 4-1 per cent, ionised. The acidity of the hydrides of the elements oxygen, sulphur, selenium and tellurium therefore falls into the regular series II2Te >H2Se >H2S >II20, in inverse order to the stability.6... 

E. Wiberg and E. Amberger, Hydrides of the Elements of Main Groups I-IV, Elsevier, London, 1971. 

We see then a gradation from purely ionic to purely covalent bonding in different molecules, and this is manifest in their chemical and physical properties. Consider, for instance, the hydrides of the elements in the second horizontal row of the periodic table. Their melting and boiling points,7 where known, are given below. 

The electrophilic, character of Hg ( (XP,) atoms is further confirmed by the data in Table IX. The qualitative trend is again obvious—an increase in the nucleophilic character of the substrate molecule causes an increase in the quenching cross section. The hydrides of the elements in the same group of the periodic table exhibit increasing values with increasing atomic weight. The substitution of a methyl group in am-... 

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