Hydrides

When compounds are formed by combining other elements with hydrogen, they are known as Hydrides. These compounds are then classified on the basis of two parameters: the type of bonding which occurs and the nature of the element hydrogen bonds with.

Read on to learn the types and charge of hydrides.

What are Hydrides?

Hydrides are compounds formed when hydrogen combines with another element. Depending on the bonding and electronegativity of the other element, hydrogen may show oxidation state –1, +1, or 0. Since these compounds contain one other type of element apart from hydrogen itself, they come under binary compounds. The other element’s (which combines with H) electronegativity and position in the periodic table ends up deciding the nature of the compound.

Classification of Hydrides

Based on the bonding which happens between hydrogen and the other element, the hydrides are classified.

1. Ionic Hydrides (or Saline Hydrides):

These types of hydrides are formed when the elements from Group 1 (alkali metals) and Group 2 (alkaline earth metals, except Mg, Be) form electrovalent or ionic bonds with H.

It has H– ions
It has properties of:

In molten state, conducting
They have white crystalline solid appearance.
With water, they react violently, producing hydrogen gas.
These act as strong reducing agents

Example: NaH, CaH₂

2. Covalent Hydrides (or Molecular Hydrides):

These types of hydrides are from when the elements from p-block form covalent bonds (by sharing electrons) with H.

In covalent hydrides, H usually has an oxidation state of +1 when bonded to more electronegative atoms (O, N, F), and –1 when bonded to less electronegative atoms (like in CH₄, SiH₄).
They exist as molecules which are discrete.
The properties:

In their nature, they are volatile
Sometimes, the molecules also show hydrogen bonding (like in H₂O, HF, etc).
On the basis of the geometry, the polarity of the molecule is also decided.

Example:

Electron-deficient hydrides (e.g., B₂H₆)
Electron-precise hydrides (e.g., CH₄, NH₃)
Electron-rich hydrides (e.g., H₂O, HF)

3. Metallic Hydrides (or Interstitial Hydrides):

These types of hydrides are formed when the transition elements (d-block and f-block) form metallic bonds with H. Most common with transition metals of 3d, 4d, 5d series.

It has neutral H ions (because of the lattice)
The properties are as follows:

As these compounds will be metallic in nature, they conduct both heat and electricity.
Hard → high melting points
These compounds are such that their ratio cannot be expressed in whole numbers, thus making them non-stoichiometric.

Example: TiH₂, LaH₃

4. Complex Hydrides:

Hydrogen, when forming bonds with highly electropositive metals, when other elements are also present, comes under complex hydrides.

It has complex anions like [BH₄]⁻, etc.
Properties:

They are used as reducing agents.
They are also used in organic synthesis.

Examples: LiAlH₄, NaBH₄

Formal Charge in Hydrides

The formal changes on H for different types are-

Ionic Hydrides: –1
Covalent Hydrides: +1 (when they are bonded to high electronegative elements like O, F, etc.)
Metallic Hydrides: Neutral (since they mostly act as an impurity)

Summing Up

We have broadly classified Hydrides into ionic, covalent, metallic, and complex types. The ionic hydrides are salts of H⁻. The covalent hydrides are molecular compounds. The metallic hydrides are interstitial solid solutions, and lastly the complex hydrides act as reducing agents.