[Chemistry Class Notes] on Covalent Hydrides Pdf for Exam

Dihydrogen, which is better known as H2, is the most abundant element in the world and it can combine with almost anything and form carbohydrates, proteins, hydrides, hydrocarbons and many other compounds. Dihydrogen or H2 combines with almost every element on earth, except noble gasses, and forms binary compounds called hydrides under certain conditions. MgH2, B2H6 are two examples of hydrides. These hydrides are broadly classified into three categories:

• Ionic or saline or salt like hydrides

• Covalent or molecular hydrides

• Metallic or non-stoichiometric hydrides

Ionic hydrides

These compounds of H2 are mostly formed with s-block elements which are highly electropositive in nature. Although lighter metal hydrides such as LiH, BeH2 have significant covalent character.

LiH is unreactive at a moderate temperature with O2 or Cl2, so it is used in the synthesis of other useful hydrides.

Covalent hydrides

H2 forms molecular bonds with most of the p-block elements and the most common compounds are H2O, NH3, CH4 and HF.

Covalent hydrides are further divided into three categories according to the relative numbers of electrons and bonds in their Lewis structure:

(i) electron-deficient – all the group 13 elements form electron deficient compounds with hydrogen and act as electron acceptor (Lewis acids), eg, B2H6,

(ii) electron-precise – all group 14 elements form electron precise compounds and see tetrahedral in geometry, eg, CH4,

(iii) electron-rich hydrides – elements from groups 15 through 17 form such bonds that have excess electrons which are present as lone pairs, eg, NH3 where 1 lone electron is present.

Metallic hydrides

Hydrides formed by many elements of d and f-block with H2 are called metallic hydrides. Elements from groups 7,8 and 9 do not form bonds with H2, and only Cr from group 6 forms a bond. Some of the metals can accommodate a large number of hydrogen making them highly potential for hydrogen storage, eg, Pt, Pd. In this type of hydride, hydrogen occupies the metal lattices in the interstitial spaces.