Hydride
Table of Contents
A hydride ion is the anion of hydrogen, represented as H–. It forms when a hydrogen atom gains an electron, producing a negatively charged species. In compounds, this hydride ion bonds with metals or other electropositive elements, generating a broad class of substances collectively known as hydrides. [1-2]
Hydrides are primarily used as:
- strong reducing agents in chemical synthesis
- reagents for generating hydrogen gas
- materials for hydrogen storage and transport
- intermediates in inorganic, organic, and industrial chemistry
For instance, sodium borohydride (NaBH4) is applicable in both academic and industrial laboratories as a selective reducing agent.
Types [1,3]
1. Ionic Hydrides
Ionic or saline hydrides form when hydrogen reacts with highly electropositive alkali metals (Li, Na, K) and most alkaline-earth metals (Ca, Sr, Ba). The exception is Be and Mg, whose hydrides do not display pure ionic character.
Examples:
- Lithium hydride (LiH)
- Sodium hydride (NaH)
- Calcium hydride (CaH2)
2. Covalent Hydrides
Covalent or molecular hydrides form when hydrogen bonds covalently to nonmetals or metalloids, typically elements from Groups 13–17. The electronegativity difference between hydrogen and the bonded element is responsible for many of their properties.
Examples by group:
- Group 13: Diborane (B2H6)
- Group 14: Methane (CH4)
- Group 15: Ammonia (NH3)
- Group 16: Water (H2O)
- Group 17: Hydrogen chloride (HCl)
3. Metallic Hydrides
Metallic or interstitial hydrides form when hydrogen atoms occupy interstitial sites within a metal’s crystal lattice. Many transition-metal hydrides are nonstoichiometric and form by hydrogen occupying interstitial sites.
Examples:
4. Complex Hydrides
Complex hydrides contain polyatomic hydride-bearing anions such as borohydride (BH4–) and alanates (AlH4–). They differ from simple hydrides in having distinct, polyatomic hydride-containing anions, rather than just interstitial H atoms in a metal lattice.
Examples:
Physical Properties [1]
| Type of Hydride | Physical State | Electrical Conductivity | Thermal Stability |
|---|---|---|---|
| IonicHydrides | Crystalline solids with high melting points | Non-conductive as solids; conductive when molten | Stable at moderate temperatures; decompose upon heating |
| CovalentHydrides | Solids, liquids, or gases depending on molecular size and polarity | Generally non-conductive | Highly variable; some stable, others decompose readily |
| MetallicHydrides | Metallic solids resembling the parent metal | Good electrical conductivity (metal-like) | Generally thermally stable; release hydrogen at elevated temperatures |
| ComplexHydrides | Crystalline solids containing polyatomic hydride anions | Non-conductive; may conduct ions at elevated temperatures | Often thermally stable but decompose to release hydrogen upon heating |
Preparation [4,5]
1. Direct Combination with Hydrogen
Many ionic and metallic hydrides form by reacting hydrogen gas with the elemental metal at elevated temperatures.
i. 2 Na (s) + H2 (g) → 2 NaH (s)
ii. Ca (s) + H2 (g) → CaH2 (s)
iii. Ti (s) + H2 (g) → TiH2 (s)
2. Metathesis (Hydride Exchange) Method
This method applies to complex hydrides.
i. Lithium aluminum hydride (LiAlH4) is prepared in the laboratory by reacting lithium hydride with aluminum chloride in dry ether:
4 LiH + AlCl3 → LiAlH4 + 3 LiCl (in the presence of diethyl ether)
ii. In the Brown–Schlesinger process, trimethyl borate reacts with sodium hydride at ~250–270 °C to form sodium boron hydride (NaBH4):
4 NaH + B(OCH3)3 → NaBH4 + 3 NaOCH3
Chemical Reactions [5-7]
1. Reaction with Water
Many ionic hydrides (LiH, NaH, CaH2) react vigorously with water because the hydride ion is a strong base that abstracts a proton, forming hydrogen gas. However, precautions are necessary as these reactions evolve flammable H2.
General reaction:
H– + H2O → H2 (g) + OH–
Examples:
i. LiH (s) + H2O (l) → LiOH (aq) + H2 (g)
ii. NaH (s) + H2O (l) → NaOH (aq) + H2 (g)
iii. CaH2 (s) + 2 H2O (l) → Ca(OH)2 (s) + 2 H2 (g)
Covalent hydrides show more variable aqueous behavior. Boranes hydrolyze readily, whereas methane and ammonia do not react with water under normal conditions.
2. Hydrogen Evolution and Storage
Metallic and complex hydrides can reversibly absorb and desorb hydrogen according to temperature and pressure. This property underpins their role in hydrogen-storage technologies.
Examples:
i. Hydrogen absorption: Pd (s) + xH2 (g) → PdHx (s)
ii. Hydrogen release: PdHx (s) → Pd (s) + xH2 (g)
3. Hydride Reduction
Complex hydrides function as prominent hydride donors in both inorganic and organic chemistry.
Examples:
i. LiAlH4 reduces aldehydes to primary alcohols:
R–CHO + LiAlH4 → R–CH2OH
ii. NaBH4 reduces silver ions to metallic silver:
2 AgNO3 + 2 NaBH4 → 2 Ag + B2H6 + 2 NaNO3 + H2
Stoichiometry and volatile by-products vary with solvent, pH, and temperature.
Hydrides constitute a structurally and functionally diverse class of compounds whose chemical behavior depends on the nature of the hydrogen species they contain. Their ability to act as bases, nucleophiles, reducing agents, or hydrogen reservoirs links them to essential processes across inorganic synthesis, organic transformations, catalysis, metallurgy, and emerging energy technologies.
