Hydrogen combines with nearly every other element to form a diverse group of compounds known as hydrides. These compounds exhibit a wide array of physical and chemical properties depending on the element to which hydrogen is bonded. Ranging from volatile gases to crystalline solids and metallic alloys, hydrides are a foundational class of chemical substances. They have applications spanning industrial synthesis, energy storage, and modern technology.
Defining the Hydride Concept
A hydride is a compound where hydrogen is chemically bound to another element. The chemical definition relies on the oxidation state of hydrogen. While hydrogen usually carries a positive charge (+1) in most compounds, the term “hydride” is applied when hydrogen is bonded to a less electronegative element. This forces the hydrogen atom to gain an electron, resulting in the formation of the hydride ion (\(\text{H}^-\)) with an oxidation state of -1.
The presence of the \(\text{H}^-\) ion gives hydrides their unique chemical reactivity, often allowing them to act as a strong base or a powerful reducing agent. Hydrides are generally represented by the formula \(\text{EH}_x\) or \(\text{E}_m\text{H}_n\). Nearly all elements in the periodic table, except for the noble gases and a few others, are capable of forming some type of hydride.
Classification by Bonding Type
Hydrides are broadly classified into three main types based on the nature of the chemical bond between hydrogen and the other element. The bonding type dictates the compound’s physical state, melting point, electrical conductivity, and overall chemical behavior. These distinct categories are ionic, covalent, and metallic hydrides.
Ionic (Saline) Hydrides
Ionic hydrides, or saline hydrides, form when hydrogen reacts with highly electropositive metals, primarily those from Group 1 and some from Group 2. The large difference in electronegativity causes the metal to transfer an electron to the hydrogen atom, creating an ionic bond between the positive metal cation (\(\text{M}^+\)) and the hydride ion (\(\text{H}^-\)).
Compounds like sodium hydride (\(\text{NaH}\)) and calcium hydride (\(\text{CaH}_2\)) are typically white, crystalline solids with high melting and boiling points. These properties result from the strong electrostatic forces holding the ions together. Saline hydrides react vigorously with water, instantly producing hydrogen gas and a metal hydroxide, making them useful as powerful bases and portable hydrogen sources.
Covalent (Molecular) Hydrides
Covalent hydrides form when hydrogen bonds with non-metallic elements, mainly those in Groups 13 through 17. Electrons are shared between the atoms, resulting in a covalent bond where the hydrogen atom does not fully transfer an electron. Since the electronegativities are comparable, these compounds exist as discrete molecules.
Molecular hydrides are characterized by low melting and boiling points, often existing as gases or volatile liquids at room temperature, such as methane (\(\text{CH}_4\)) and ammonia (\(\text{NH}_3\)). Their physical properties are determined by weak intermolecular forces, though hydrogen bonding in compounds like water and ammonia leads to higher boiling points. They are classified based on the number of electrons around the central atom: electron-deficient (boranes), electron-precise (methane), or electron-rich (ammonia).
Metallic (Interstitial) Hydrides
Metallic hydrides form when hydrogen interacts with transition metals and inner transition metals. Hydrogen atoms do not form traditional bonds but instead occupy the tiny spaces, or interstices, within the metal’s crystal lattice structure. They were historically referred to as interstitial hydrides.
These compounds retain many properties of the parent metal, often exhibiting metallic luster and electrical conductivity. Many metallic hydrides are non-stoichiometric, meaning the ratio of metal to hydrogen atoms is variable (e.g., \(\text{TiH}_{1.7}\) or \(\text{PdH}_x\)). The ability of these metals to reversibly absorb and release large volumes of hydrogen makes them important for storage technology.
Hydrides in Real-World Use
The diverse chemical properties of hydrides have led to their integration into a wide range of industrial and technological applications. The function of a hydride in a real-world setting is dictated by its bonding type, with the most important uses centering on chemical synthesis and energy technology.
Chemical Synthesis and Reduction
Hydrides are widely used as powerful reducing agents in chemical reactions. Complex ionic hydrides, such as lithium aluminum hydride (\(\text{LiAlH}_4\)) and sodium borohydride (\(\text{NaBH}_4\)), are valued in organic chemistry and pharmaceutical manufacturing. They function by efficiently donating the hydride ion (\(\text{H}^-\)) to another molecule, facilitating the controlled reduction of functional groups.
Hydrides are also used as specialized chemical reagents for removing trace water. Calcium hydride (\(\text{CaH}_2\)), for example, is used as an effective drying agent, or desiccant, for organic solvents. It reacts readily with water to produce hydrogen gas and a metal hydroxide, removing moisture without introducing other contaminants.
Energy Storage
In the energy sector, metallic hydrides play a growing role, particularly in hydrogen fuel storage. Metals like palladium, titanium, and magnesium alloys can safely absorb hydrogen at low pressures and release it when heated. This offers a high-density, solid-state storage medium for use in fuel cell vehicles and stationary power systems. Furthermore, metal hydrides form the foundation of nickel-metal hydride (\(\text{Ni-MH}\)) rechargeable batteries, which are used in consumer electronics and hybrid electric vehicles.