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Covalent Hydrides

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 +3H   →    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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