Classification of Hydrides

Hydride Definition:

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 e.g., MgH₂ or B₂H₆.

The hydrides are classified into three categories:

1. Ionic

2. Covalent or molecular hydrides

3. Metallic or non-stoichiometric hydrides

1. Ionic or Saline Hydrides:

Potassium hydride

What is an Ionic Hydride?

These are stoichiometric compounds of dihydrogen formed with most of the s-block elements which are highly electropositive in character.

How they are formed?

· They are formed by the transfer of electrons from the metals to the hydrogen atoms.

· It can be only formed from highly electropositive element of group 1 and group 2 (except Be and Mg).

General Expression of Preparation:

For the hydrides of group A,

2M (g) + H₂ (s) 2MH (g)

For the hydrides of group B,

M (g) + H₂ (g) MH₂ (g)

Properties of Ionic Hydrides:

· 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.

· They are thermally very stable.

· They have high melting and boiling point.

· They are soluble in water.

· They are insoluble in organic compounds.

· They are conductors (electrolytes) of electricity.

· On electrolysis liberate dihydrogen gas at anode which confirms the existence of H– ionc or saline or salt like hydrides.

Uses of Ionic Hydrides:

· When ionic hydrides react with water the metal combines with the OH from the water to form a hydroxide salt.

· The hydrogen from the ionic hydride and the other water hydrogen combine to form the hydrogen gas.

· Saline hydrides react violently with water producing dihydrogen gas.

· On heating ionic hydrides decompose to evolve dihydrogen which ignites spontaneously therefore they are used as solid fuels.

2. Covalent or Molecular Hydride:

Boron Hydride

What is a Covalent Hydride?

Covalent hydrides are primarily compounds of hydrogen and non-metals in which the bonds are evidently electron pairs shared by atoms of comparable electronegativities. For example, most nonmetal hydrides are volatile compounds held together in the condensed state by relatively weak vanderwaal’s intermolecular interactions.

How they are formed?

· Covalent hydrides can be formed from boron (B), aluminum (Al) and gallium (Ga) of group 13 in the periodic table.

· Boron forms an extensive series of hydrides. 

Preparation:

· By direct combination of elements with dihydrogen:

H₂ (g) + S (l) H₂S (g)

· By reduction of a suitable halide with LiAlH₄ in dry ether:

SiCl₄ + LiAlH₄ SiH₄ +LiCl+ AlCl₃

· By hydrolysis of metal borides, carbides, nitrides, phosphide:

Ca₃P₂ (s) +6 H₂O (l) 3Ca(OH)₂ (aq) + 2PH₃ (g)

· By action on suitable binary compounds:

FeS + H₂SO₄ H₂S + FeSO₄

· By action of an oxo acids with NaBH₄ in aqueous solution:

4H₃AsO₃ + 3NaBH₄ 4AsH₃ + 3H₃BO₃ + 3NaOH

Properties of Covalent Hydrides:

· Covalent hydrides are usually volatile compounds having low melting and boiling point and also do not conduct electricity.

· Hydrides of group 13 (BH₃, AlH₃) do not have sufficient number of electrons to form normal covalent bond and hence are called electron deficient hydrides. They exist in polymeric forms such as B₂H₆, B₄H₁₀, etc.

· Hydrides of group 14 (CH₄, SiH₄, SnH₄, PbH₄) have exact number of electrons to form normal covalent bond and hence are called electron exact or electron precise hydrides. Their Bond length increases from CH₄ to PbH₄ as the size of the element increases from C to Pb.

· Hydrides of group 15, 16 and 17 (NH₃, PH₃, H₂O, H₂S, HF, HCl) have more electrons than required to form normal covalent bond and hence are called electron rich hydrides.

· Group 15 hydrides have one lone pair, group 16 hydrides have two lone pair, group 17 hydride have 3 lone pair of electrons.

· The hydrides of first elements of group 15, 16 and 17 have abnormally high boiling point as compared to boiling point of the hydrides of second element of each group.

· The boiling point of the hydrides of the rest of the elements of each group increases as the atomic number or the molecular mass of the hydride increases down the group.

· The lighter elements of group 14 ,15 and 16 form polynuclear hydrides in which two or more atoms of same elements are linked together. This property of self-linking of atoms is called catenation.

Uses of Covalent Hydrides:

· A particularly important segment of covalent hydrides are complex metal hydrides, powerful soluble hydrides commonly used in synthetic procedures.

· Hydrides that are soluble in common solvents are widely used in organic synthesis, particularly common are sodium borohydride (NaBH₄) and lithium aluminium hydride and hindered reagents such as DIBAL.

3. Metallic or Non-Stoichiometric Hydrides:

What is Covalent Hydride?

These are the hydrides of transition elements (except elements of group I-B and II-B). In transition elements there are small empty spaces among the atoms. Hydrogen gas adsorbs in these empty spaces to produce metallic hydrides. These hydrides are also known as Interstitial hydrides.

How they are formed?

Hydrides of this type form according to either one of two main mechanisms:

· The first mechanism involves the adsorption of dihydrogen succeeded by the cleaving of the H-H bond, the delocalisation of the hydrogen's electrons and finally, the diffusion of the protons into the metal lattice.

· The other main mechanism involves the electrolytic reduction of ionised hydrogen on the surface of the metal lattice, also followed by the diffusion of the protons into the lattice.

Preparation:

These hydrides are generally formed by,

· Transition metals of group 3, 4, 5 of d- block,

· Cr metal of group 6 and

· f-block elements.

Properties of Metallic Hydrides:

· They are generally powders or brittle solids having dark or metallic appearances.

· They are good conductors of electricity. The conductivity decreases with increase in temperature.

· They have high thermal conductivity.

· Most of these hydrides are harder than parent metals.

· They generally undergo reversible decomposition into H₂ gas and metal.

· They do not follow stoichiometric rules.

· They behave like pure metal.

· Bonding between metal and hydrogen is metallic bond.

· On heating they release hydrogen in atomic state.

· They are solids.

· They are not true chemical compounds. They are used as reducing agent in different processes.

· There properties are between ionic hydrides and covalent hydrides.

Uses of Metallic Hydrides:

· Metal hydrides are often used in fuel cell applications that use hydrogen as a fuel.

· Nickel hydrides are often found in various types of batteries particularly NiMH batteries.

· Nickel metal hydride batteries rely on hydrides of rare earth intermetallic compounds, such as lanthanum or neodymium bonded with cobalt or manganese.

· Lithium hydrides and sodium borohydride both serve as reducing agents in chemistry applications.