Preparation by Reduction with Elementary Hydrogen

The reaction of hydrogen with borides has been little studied. Evidence of diborane formation has been found at Vi400°C with the borides of magnesium, calcium, and 

5HBC12 B2H5CI + 3BCI3 or (6HBCI2 j , B2Hs + 4BCI3) 

This method is slow by modern standards, but at the time of its discovery, it greatly speeded up the laboratory preparation of diborane. Earlier workers had used a rather tortuous and wasteful—if historically important—procedure due to Stock, which involved the pyrolysis of higher boranes obtained from magnesim boride (301-303), itself only obtainable from a high-temperature fusion process (305). 

Alkyldichloroboranes (RBCI2) are easier to convert than BCI3 and will give up to 50% yield of B2H0 at 320 to 340°C with H2 under pressure over activated carbon (125). The reaction proceeds via HBCI2. 

TrialkyIboranes also undergo a thermodynamically favorable reaction with hydrogen, and yields up to approximately 75% are obtained at 150 to 200 C and 100 to 130 atm (180). 

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