Ionic, Covalent and Metallic Hydrides — Definition

Definition

Imagine hydrogen, the simplest element, trying to bond with almost every other element on the periodic table. When it does, it forms compounds called hydrides. But hydrogen is a bit of a chameleon; it can behave differently depending on who its partner is. This leads to three main 'personalities' or types of hydrides: ionic, covalent, and metallic.

First, let's talk about Ionic Hydrides. Think of these as the 'salt-like' hydrides. They form when hydrogen teams up with very electropositive elements, primarily the alkali metals (Group 1, like Lithium, Sodium, Potassium) and alkaline earth metals (Group 2, like Magnesium, Calcium, Strontium, Barium).

In these compounds, hydrogen acts like an anion, $H^-$, gaining an electron from the metal. So, you have a metal cation ($M^+$ or $M^{2+}$) and a hydride anion ($H^-$) held together by strong electrostatic forces, just like in common table salt (NaCl).

Because of these strong forces, ionic hydrides are typically solid, crystalline, non-volatile, and have high melting points. They are powerful reducing agents and react vigorously with water to produce hydrogen gas and a strong base.

For example, Sodium Hydride (NaH) is a classic ionic hydride.

Next, we have Covalent Hydrides. These are the 'molecular' hydrides. They form when hydrogen bonds with elements that are more electronegative than itself, or with elements that have similar electronegativity, primarily from the p-block elements (like Carbon, Nitrogen, Oxygen, Fluorine, Chlorine) and some s-block elements like Beryllium and Boron.

In these compounds, hydrogen shares electrons with the other element, forming covalent bonds. These hydrides exist as discrete molecules, and their properties depend heavily on the number of electrons around the central atom.

They can be gases, liquids, or low-melting solids. Covalent hydrides are further categorized into electron-deficient (like $BH_3$), electron-precise (like $CH_4$), and electron-rich (like $NH_3$, $H_2O$) based on whether the central atom has fewer, exactly enough, or more electrons than required to form simple covalent bonds and complete its octet.

Their reactivity varies widely.

Finally, there are Metallic Hydrides, also known as 'interstitial' hydrides. These are formed by many d-block and f-block elements (transition metals and inner transition metals). Unlike the other two types, these are often non-stoichiometric, meaning the ratio of hydrogen to the metal isn't a simple whole number (e.

g., $TiH_{1.7}$). This is because hydrogen atoms occupy the interstitial sites (small gaps) within the metal lattice, rather than forming distinct chemical bonds in the traditional sense. They retain metallic properties like electrical conductivity and luster, though often reduced compared to the parent metal.

They are typically hard solids and are known for their ability to absorb large volumes of hydrogen, making them potential candidates for hydrogen storage materials. Palladium hydride ($PdH_x$) is a well-known example.