Diborane cannot be synthesized directly from its elements as it has a positive enthalpy and a positive free energy of formation. ΔG_f ^o. This may be attributed to very strong bonds in elemental boron and H₂ which will have to be overcome before forming boron hydrides.
(1)    Diborane can be easily prepared in small quantities on a laboratory scale by the reaction iodine on sodium borohydride (Na BH₄) in diglyme a high boiling ether. [(CH₂ O CH₂ CH₂)₂O].
             
                 diglyme
2NaBH₄ + I₂  →  B₂H₆ + 2NaI + H₂
                              (98% yield)

(2)    By the action of phosphoric acid on magnesium boride.
                                                              
                                                             (heat)
Mg₃B₂ + H₃PO₄ → (Mixture of boranes)  →  B₂H₆

(3)    It can be prepared by the reduction of boron halides by any one of the following methods:
(i)    Reduction of boron trifluoride with lithium hydride
           
                 Ether
8BF₃ + 6LiH  →   6LiBF₄ + B2H₆

(ii)    Reduction of boron trichloride with lithium aluminum-hydride. This reaction gives quantitative yield

4BCI₃ + 3LiAIH₄ → 2B₂H₆ + 3LiCI + 3AICI₃

(iii)    Reduction of boron trichloride vapours with molecular hydrogen by passing a silent electric discharge at low pressure

2BCI₃ + 6H₂  →  B₂H₆ + 6HCI

(iv)    It is quantitatively obtained by reaction of sodium borohydride with boron trifluoride

4BF₃ + 3NaBH₄  →  3NaBF₄ + 2B₂H₆

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