These are compounds of non-metals and some non-transition metals like Sn, As, Sb with hydrogen when hydrogen atoms are predominantly covalently bonded to the element. These compounds are formed by elements of main groups 13 to 17 of the periodic Table. The common examples of this variety are : hydrides of Be , B ,Si, Ge , Sn, Pb, N, P, As , Sb, O, S,Se and halogens.

Preparation of Covalent Hydrides

Some typical methods of synthesis of covalent hydrides are given on next page.   

(a)    Hydrides of N, S and halogens are obtained by the action of H₂ on the element under appropriate conditions :

                                                 N₂ +3H₂    →    2NH₃
                                                 F₂ + H₂      →     2HF
                                                   S + H₂      →     H₂S

(b)    Hydrides of B, N, P, S, etc are prepared by the reaction of metal salts with water of acid.

                                     Ca₃P₂ + 6H₂O     →     3Ca(OH)₂+ 2PH₃
                                      FeS + 2HCI          →        FecI₂ + H₂S
                                      Mg₃N₂ + 6H₂O    →      3Mg(OH)₂ + 2NH₃
                                       Ca₃P₂ + 6HCI     →      3CaCI₂ + 2PH₃
                                     2NaBH₄ + 2HCI    →     B₂H₆ + 2H₂ + 2NaCI

(c)    Complex  hydrides are widely used of the synthesis of covalent hydrides

                                     LiAIH₄ + SiCI₄    →    SiH₄ + LiCI + AICI₃
              3NaBH₄ + H₃AsO₃ + 3HCI     →    4AsH₃ +3NaCI + H₃BO₃
                                  3LiAIH₄ + 4BCI₃    →        2B₂H₆ + 3LiCI + 3AICI₃

(d)    Beryllium hydride  cannot be prepared by the reaction of BeCI₂ with LiAIH₄ but is easily formed by its reaction with saline hydrides.

                                      BeCI₂ + 2NaH   →    BeH₂ + 2NaCI

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