Ionic, Covalent and Metallic Hydrides — Core Principles

Core Principles

Hydrides are binary compounds of hydrogen with other elements, classified into three main types based on bonding and properties. Ionic hydrides (Group 1 & 2 metals like NaH, CaH2) are salt-like, formed by electron transfer to hydrogen ($H^-$).

They are strong reducing agents, react violently with water to produce $H_2$ and a base, and are non-conductive solids but conductive in molten state. Covalent hydrides (p-block elements like $CH_4$, $NH_3$, $H_2O$) are molecular, formed by electron sharing.

They are classified as electron-deficient (e.g., $B_2H_6$), electron-precise (e.g., $CH_4$), or electron-rich (e.g., $NH_3$) based on electron count around the central atom, influencing their geometry and intermolecular forces.

Their properties vary widely from acidic to basic. Metallic hydrides (d-block & f-block elements like $TiH_x$, $PdH_x$) are interstitial, non-stoichiometric compounds where hydrogen occupies voids in the metal lattice.

They retain metallic properties, are hard solids, and are important for hydrogen storage. The 'hydride gap' refers to the absence of hydrides for Group 7, 8, 9 metals.

Important Differences

vs Ionic, Covalent, and Metallic Hydrides

Aspect	This Topic	Ionic, Covalent, and Metallic Hydrides
Elements Involved	Ionic Hydrides (e.g., NaH, CaH2)	Covalent Hydrides (e.g., CH4, NH3, H2O)
Bonding Nature	Predominantly ionic (electron transfer, $M^+H^-$)	Covalent (electron sharing)
Physical State (at RT)	Crystalline solids, high melting points	Gases, liquids, or low-melting solids
Electrical Conductivity	Non-conductive in solid state; conductive in molten state	Non-conductive (insulators)
Stoichiometry	Stoichiometric (fixed, simple whole-number ratio)	Stoichiometric (fixed, simple whole-number ratio)
Reactivity with Water	Vigorous reaction, produces $H_2$ gas and strong base	Varies (e.g., $H_2O$ miscible, $HCl$ acidic, $CH_4$ inert)
Reducing/Oxidizing Nature	Strong reducing agents ($H^-$)	Varies (e.g., $NH_3$ weak reducing, $H_2O$ mild oxidizing/reducing)
Examples	$LiH, NaH, CaH_2, SrH_2$	$CH_4, NH_3, H_2O, HF, SiH_4, H_2S$

The three main types of hydrides—ionic, covalent, and metallic—are fundamentally distinguished by the nature of their chemical bonding, which in turn dictates their macroscopic properties and reactivity. Ionic hydrides, formed with highly electropositive metals, involve electron transfer to hydrogen, resulting in salt-like solids with high melting points and strong reducing capabilities. Covalent hydrides, formed with p-block elements, involve electron sharing, leading to molecular compounds with diverse physical states and chemical behaviors (acidic, basic, or neutral). Metallic hydrides, formed with d- and f-block metals, are characterized by hydrogen occupying interstitial sites, resulting in non-stoichiometric compounds that retain metallic properties and are crucial for hydrogen storage.