How do metallic hydrides differ from saline hydrides?

Metallic and saline (ionic) hydrides differ primarily in their chemical bonding, composition, and physical properties.

Saline (Ionic) Hydrides

These compounds are formed by alkali metals and alkaline-earth metals. They are defined by the presence of hydrogen as a negatively charged hydride ion ($H^{-}$).1
* Structure: They are white crystalline solids that exhibit ionic bonding.1
* Reactivity: The hydride ion acts as a strong base, reacting instantly and quantitatively with water to produce hydrogen gas.1
* Utility: Due to this vigorous reaction, they serve as portable sources of hydrogen.1

Metallic Hydrides

Formed by transition metals and inner transition metals, these are often called interstitial or alloylike hydrides.23
* Structure: They possess metallic characteristics, such as lustre and electrical conductivity, resulting from relatively free electron movement. Unlike the fixed composition of saline hydrides, many metallic hydrides are nonstoichiometric, meaning they have variable compositions.2
* Nature: They are considered intermediate between salts and alloys.2
* Reactivity: They are generally stable in air at ambient temperatures but can be reactive when heated.2