- Ionic or salt like or saline hydrides
- Covalent or molecular hydrides
- Metallic or interstitial hydrides
Ionic hydrides or saline hydrides
The combinations of hydrogen with metals, which have low electronegativity values and are electropositive with respect to hydrogen, form ionic hydrides. Elements of group IA (alkali metals), group IIA (alkaline Earth metals with the exception of Be and Mg) and lanthanum form these compounds when the transfer of electrons from metals to hydrogen atom takes place. The hydrogen atom exists as
H- ion. For example, lithium hydride (Li+H-), calcium hydride (Ca2+H2-), sodium hydride (Na+H-), etc.Ionic hydrides are prepared by the direct combination of the metals with hydrogen at high temperatures of 750oC.
Characteristics of ionic hydrides
- Ionic hydrides are white or light grey crystalline solids. They behave like salts.
- They have high melting and boiling points.
- They conduct electricity in fused state liberating hydrogen at the anode. The negative charge of the hydrogen on them is proved by electrolysis of molten lithium hydride at 700oC when lithium metal deposits at the cathode and hydrogen is liberated at the anode.
- Their densities are higher than those of metals.
- The thermal stability of the alkali metal hydrides decreases from LiH to CsH. LiH, CaH2 and SrH2 are the most stable hydrides of this class; others undergo thermal decomposition above 500oC.
- The ionic hydrides are oxidized by air to form metal oxide and water.
RbH and CsH burn spontaneously at room temperature.
- They are hydrolyzed by water with the formation of hydroxide and liberation of hydrogen.
- They react with protonic solvents liberating hydrogen.
- They act as powerful reducing agents, at high temperatures. For example, NaH reduces CO and CO2 to formates on heating.
CO2 + NaH
They reduce sulphates to sulphides,
They also reduce some halides and oxides as:
They are capable of reducing organic acids to alcohols.
- Some hydrides are used to prepare complex hydrides of other metals. These complex hydrides also act as reducing agents.
Uses of ionic hydrides
Ionic hydrides and their complexes are used as reducing agents. They evolve hydrogen when heated. Hence they are used as solid fuels as they ignite spontaneously.
Molecular or covalent hydrides
Molecular hydrides are formed by the combination of elements of comparatively higher electronegativity as of p block elements. These bonds are mostly covalent in character with partly ionic character too (for example, in HF). These hydrides, therefore have molecular lattices held together by weak van der Waals forces. In some cases, hydrogen bonds are also formed. The general formula for covalent hydrides is XH8-n where X stands for the symbol of a metal and 'n' is the group number.
Preparation of covalent hydrides
These hydrides are prepared by the following methods:
- By direct combination of elements, e.g.,
- By hydrolysis of metal borides, carbides, nitrides, phosphides etc.
- They are formed through the reduction of certain compounds by nascent hydrogen or by using reducing agents like LiAlH4 (ether solution).
General characteristics of covalent hydrides
- Molecular hydrides are soft.
- They are generally, volatile in nature and have low melting and boiling points.
- They have low electrical conductivity.
- The electronegativity difference between hydrogen and the atom bonded to it, determines the properties of covalent hydrides. When moving from left to right along a given row in the periodic table hydrides become increasingly acidic in character. For example NH3 is a weak base, H2O is neutral and HF is acidic. Similarly in the next row while PH3 is a weak base, H2S is a weak acid and HCl is highly acidic.
The hydrides of Group III (e.g., BH3 and AlH3) are electron deficient compounds and exist in polymerised forms [e.g., B2H6 and (AlH3)n]. Carbon forms polynuclear hydrides wherein numerous hydrocarbons are generated (two or more atoms of the element) and directly bond to each other as well as to hydrogen. Silicon and other non-metal elements also show this, property to a limited extent.
Metallic or interstitial hydrides
Metallic hydrides are formed by most of the 'd' block elements (i.e., transition elements), on reacting with hydrogen. Hydrogen exists in the atomic rather than ionic form. Due to small size of hydrogen atoms when compared to the metal atoms, hydrogen atoms occupy interstitial positions in the metal lattices. Thus these are interstitial compounds and some workers regard them nearly as solid solutions. Beryllium and magnesium of 's' block elements also form metallic hydrides.
When hydrogen is directly adsorbed into the interstices of transition metal lattices at appropriate temperatures, metallic hydrides are formed. For example, palladium metal adsorbs hydrogen to form palladium hydride. In some cases, the metals are used as cathodes in the electrolysis of their aqueous solutions to ensure that metals adsorb hydrogen during electrolysis. PdH2 is prepared by this method.Metallic or interstitial hydrides compounds are non-stoichiometric, and their composition varies with temperature and pressure. To illustrate, the compositions of titanium and zirconium hydrides are represented as TiH1.7, and ZrH1.9 respectively. The density of each compound is less than that of the metal itself and their properties are not much different from their respective metal i.e. they give out hydrogen easily and are strong reducing agents, suggesting that the presence of hydrogen in its atomic state. These compounds are used as industrial reducing agents.
Non-stoichiometric interstitial hydrides are formed by the 'f' block elements (i.e. lanthanides and actinides). These hydrides have lower densities than their respective metals, e.g. LaH2.76, CeH2.69, TbH3.07 .
