Dihydrogen, under certain reaction conditions, combines with almost all elements, except noble gases, to form binary compounds, called hydrides.
If ‘E’ is the symbol of an element then hydride can be expressed as EHx (e.g., MgH2) or EmHn (e.g., B2H6).
The hydrides are classified into three categories :
(i) Ionic or saline or saltlike hydrides
(ii) Covalent or molecular hydrides
(iii) Metallic or non-stoichiometric hydrides
9.5.1 Ionic or Saline Hydrides
These are stoichiometric compounds of dihydrogen formed with most of the s-block elements which are highly electropositive in character.
However, significant covalent character is found in the lighter metal hydrides such as LiH, BeH2 and MgH2. In fact BeH2 and MgH2 are polymeric in structure.
The ionic hydrides are crystalline, non-volatile and nonconducting in solid state. However, their melts conduct electricity and on electrolysis liberate dihydrogen gas at anode, which confirms the existence of H– ion.
2H- (melt) → H2 + 2e-
Saline hydrides react violently with water producing dihydrogen gas.
NaH (s) + H2O (aq) → NaOH (aq) + H2 (g)
Lithium hydride is rather unreactive at moderate temperatures with O2 or Cl2. It is, therefore, used in the synthesis of other useful hydrides, e.g.,
8LiH + Al2Cl6 → 2LiAlH4 + 6LiCl
2LiH + B2H6 → 2LiBH4
9.5.2 Covalent or Molecular Hydride
Dihydrogen forms molecular compounds with most of the p-block elements. Most familiar examples are CH4, NH3, H2O and HF.
For convenience hydrogen compounds of nonmetals have also been considered as hydrides. Being covalent, they are volatile compounds
Molecular hydrides are further classified according to the relative numbers of electrons and bonds in their Lewis structure into : (i) electron-deficient, (ii) electron-precise, and (iii) electron-rich hydrides.
An electron-deficient hydride, as the name suggests, has too few electrons for writing its conventional Lewis structure. Diborane (B2H6) is an example.
In fact all elements of group 13 will form electron-deficient compounds. What do you expect from their behaviour? They act as Lewis acids i.e., electron acceptors.
Electron-precise compounds have the required number of electrons to write their conventional Lewis structures. All elements of group 14 form such compounds (e.g., CH4) which are tetrahedral in geometry.
Electron-rich hydrides have excess electrons which are present as lone pairs. Elements of group 15-17 form such compounds. (NH3 has 1- lone pair, H2O– 2 and HF–3 lone pairs).
What do you expect from the behaviour of such compounds ? They will behave as Lewis bases i.e., electron donors. The presence of lone pairs on highly electronegative atoms like N, O and F in hydrides results in hydrogen bond formation between the molecules.
This leads to the association of molecules.
9.5.3 Metallic or Non-stoichiometric (or Interstitial ) Hydrides
These are formed by many d-block and f-block elements. However, the metals of group 7, 8 and 9 do not form hydride. Even from group 6, only chromium forms CrH.
These hydrides conduct heat and electricity though not as efficiently as their parent metals do. Unlike saline hydrides, they are almost always nonstoichiometric, being deficient in hydrogen.
For example, LaH2.87, YbH2.55, TiH1.5–1.8, ZrH1.3–1.75, VH0.56, NiH0.6–0.7, PdH0.6–0.8 etc. In such hydrides, the law of constant composition does not hold good.
Earlier it was thought that in these hydrides, hydrogen occupies interstices in the metal lattice producing distortion without any change in its type. Consequently, they were termed as interstitial hydrides.
However, recent studies have shown that except for hydrides of Ni, Pd, Ce and Ac, other hydrides of this class have lattice different from that of the parent metal.
The property of absorption of hydrogen on transition metals is widely used in catalytic reduction / hydrogenation reactions for the preparation of large number of compounds.
Some of the metals (e.g., Pd, Pt) can accommodate a very large volume of hydrogen and, therefore, can be used as its storage media. This property has high potential for hydrogen storage and as a source of energy.